There is no strict timeframe that one has to follow to clean their PC. It all depends on where they have placed the system and how is it being used.
Dust will cause poor heat radiation thereby straining all your components. If you have the time, you can very well clean it using your vaccum cleaner once a week. As stated earlier use the sucking option.
With regards to TIM.. you can ask any computer hardware stores for IPA, they cost around Rs50 for 500ml here in chennai. Its a good alternative to clean your motherboard, RAM everything, but you should allow it to evaporate completely before you put back your components to use.
Happy cleaning