For me that answer is easy.
My tanks all have an "earthy" smell. Not bad, sort of like clean mud? Outdoorsy without being nasty! If it smells bad to you, look for a dead fish, or even just fish food stuck in the rim or the filter can make a tank really smelly! If you were not using a siphon before the water will be really smelly, so just the siphoning of the gravel alone might clear up that problem. Siphoning of the gravel is a must!
How often you need to change your water depends on the size of the tank and how stocked (or over-stocked!) it is.
My 12 gallon gets 20% of the water changed weekly, using a siphon, so the gravel gets cleaned at the same time. I also change the filter media at that time, and give the interior a quick wipe off. It is slightly overstocked, so I stay on top of that, and never had a problem.
My 3 gallon gets 1 gallon changed out every 10 days or so, it's understocked right now so it's not very dirty. I follow the same procedure using a siphon to clean the gravel and I change the filter media.
My one gallon that houses our betta gets a 100% water change once every 10 days also.
The best way to figure this out is to test with a liquid kit and watch what happens every other day or so. That will give you idea how often you need to change your water. The smaller the tank, the more maintenance it will need really.