when you start thinking that it needs cleaning, then it's true, so clean them

2 months once.
You should start cleaning it when you found it sticky.

keycaps? if you want an easy way, get polident or denture tabs.
put keycaps into a basin and put 1 or 2 tabs inside, depending on how slimy and greasy your caps is, after that fill water until the water level just covers all the keycaps, you don't want more.
water temp should be room temperature or just slightly warm a bit, too hot might risk deforming the keycaps.
leave them for 30 minutes, drain and rinse, leave it to dry and u're done.

Use polident, the method is explained by justnits.

Why polident/denture tabs?
- Anti-bacterial cleaning system
- Remove odour-causing bacteria
- Restores natural whiteness of keycaps
- Cleans where brushing can miss
- Cleans surface pores

y u no trust me