Well I believe our information is correct obviously
Changing all the bedding monthly is not common I think - it would cost a small fortune, and completely stress out the hamster.
If you've seen the cage cleaning tips article, we recommend spot cleaning (as you do) and then doing say a half or third substrate clean every 2 or 3 months as well (it varies how long depending on how messed up a hamster's cage gets and how big it is). The more substrate you have, the less often you need to do a major clean. I tend to find two thirds remain clean anyway and it's only the sleeping end that gets messed up.
I think people who replace all the substrate monthly, maybe don't have that much substrate in possibly. The only unhygienic thing is their pee - provided the cage or enclosure is well ventilated and not subject to condensation (which could create mould). Hamsters are actually very clean little things and groom and wash themselves a lot. Even their poops aren't really dirty or smelly, they are like little hard seeds and they can actually eat them (which is normal - they have two stomachs and can redigest nutrients from their poops). So unless they start taking over in a big way you don't need to worry about those much and they can be spot cleaned out as and when needed. If you remove all of them, the hamster will feel you have robbed him or her of emergency food supplies!
If it is an extremely hot country most of the time, however, you may need to keep an eye on things a bit more often - maybe every two months. A wheel and any other cage items can be cleaned separately, as and when needed.
Personally I think it is our human instinct to want to clean everything all at once - which might give satisfaction but isn't good for the hamster's stress.
