Absolutely nothing bad will happen except you may pay more (or less) money over the life of the printer than the manufacturer thinks you should. The recommended monthly page volume is simply a guide to help potential purchasers spec the appropriate printer. (Of course, I would never publicly accuse a printer manufacturer of trying to steer very low volume users to much more expensive inkjet based printers – even though inkjet printers need to be run regularly or the ink heads clog – unlike lasers)
As a rule the smaller the volume the higher the cost of consumables but the lower the upfront cost. You will also generally find that printers with lower volumes (like this one) are slower and don’t have as much functionality. They are also likely to be smaller and hold less paper. Their upfront cost is less. Their consumables are more.
I am drawing on substantially more evidence than this, but here are a couple of anecdotes for you:
As it happens, I have a high end brother printer – MFC-9550CDW – which I picked up as a bargain to mainly do double-sided scanning and an occasional color print without DRM lock-in. This printer is rated at up to 75000/80000 pages per month with a recommended volume of 6000 pages per month. The printer might do 100 pages per month. It’s worked like a dream since I got it (second hand) well over a year ago.
I had a Samsung laser printer/scanner – again used for scanning – which did about 100 pages PER YEAR. It lasted many, many years until it eventually ran out of toner and when it was not viable to buy replacement toner.
The short answer is “nothing”. I have an old Canon LBP-810 that was made in Japan in early two-thousand-something. There are years when I don’t print a single page using it as I have an access to a huge floor-standing machine at office and it much easier for me to use it instead of my home Canon. But when I need to print at home the thing does the job perfectly well, just as it was 2 decades ago. The cartridge is at least 8-10 years old and it work as new.
As with a car or any mechanical device, a laser cartridge should be ‘turned over’ from time to time, as otherwise some parts which remain in constant contact for too long a period could deteriorate.
I imagine that ‘clumping’ of toner could be a problem as DrMoishe suggests, but I suspect generally only in humid climates. I live in Thailand, I know the problems which affect almost everything if you allow it to, even plastics (and laser toner is composed of minute plastic granules).
It’s sensible to print a few pages each week. Even in Thailand, and using non-manufacturer replacements I’ve never had poor toner distribution or short cartridge life (which would signal unavailable toner powder) and I print very little indeed. 150 per month sounds excessive. What’s more important is not exceeding the maximum number of pages (by too much).
I’ve had laser printers give me trouble-free printing even if they’ve been sitting there for months at a time. Toner is basically dry powdered ink, so it’s not going to fail the way an inkjet printer would if you don’t print for several months at a time. As long as it’s not exposed to excessive heat or humidity, toner has an indefinite shelf life.
That said, imaging drums can degrade over time. While this is something in the ballpark of several years, like toner, it can degrade faster if it’s stored in an excessively hot or humid environment. Of note is the fact that by its very nature, the drum is photosensitive: a drum can be damaged by excessive exposure to ambient light, such as by leaving it outside the printer for more than a few minutes without covering it with something opaque. But if it’s properly cared for and assuming it isn’t worn out through normal use, the drum should last at least five years (from my experience with Brother laser printers, which have separate toner and drum on all models), so it shouldn’t be an issue.
The Canon Color imageCLASS MF644Cdw has the toner and drum integrated into each cartridge. This means that whenever you replace a toner cartridge, you’re also replacing the imaging drum for that color. This should mitigate issues with drum degradation, as it’s unlikely that any particular cartridge will be in the printer for an extended period of time. (Most Canon laser printers use this cartridge design, known as the Single-Cartridge System. The idea is to simplify management and replacement of supplies by not having to deal with separate drums or waste toner containers, though cost per page may be a bit higher.)
The fuser and transfer belt (monochrome laser printers use a simpler transfer roller), which are often consumable supplies on high-volume printers and copiers, are designed to last the life of this printer. These parts are subject to physical wear and tear during use, but do not degrade appreciably in storage.
Any type of printer left unused is at risk of developing flats on the rubber feed rollers… I’ve not investigated in depth but it would be reasonable to assume that if a standard-sized sheet goes through the rollers don’t settle in exactly the same position.
However by and large a laser printer will probably be more tolerant of erratic use than an inkjet.
My personal experience is the toner starts forming streaks on the paper. I had an old laser printer and if not used for about 2 weeks streaks would show on the paper as if the toner was leaking onto the rollers or something. If I printed every week the streaks would not appear.