guys how often do you need to apply thermal goop in general? also is it necessary to re-apply thermal paste every time you remove the heatsink?
is it ok to clean the earlier paste with just tissue paper?
The thermal paste’s made to last a while. U need not remove the heatsink to check that and reapply the paste periodically. 
Preferably yes. Since dust and foreign particles might’ve clogged into the paste. Moreover, air-bubbles would get trapped when u place the HS (with the older paste) atop the CPU IHS and lock/fix it.
Well I use a couple of tissue papers dipped with a few drops of acetone. Some peeps prefer isopropyl alcohol (IPA in short) or even the nail polish remover. Basically a solvent to wipe out the earlier goop easily. The IHS has to be cleaned thoroughly before the thermal paste’s to be applied.
lol i didnt remove heatsink to check. but i had removed it while cleaning my cabinet.
i thought using a solvent wouldnt be really necessary, but ill just use nail polish remover.
I use the cotton swabs available in the market. They already come with isopropyl alcohol and it cleans good. 
Don’t use nail polish remover, it has many essense etc compounds, which might damage your processor ! Either use acetone, or concentrated isopropyl alcohol. Else, don’t use anything, but you won’t get a very clean surface with just tissue.
Artic Silver have very good instructions for applying and removing heat sink compounds.
Arctic Silver Incorporated - Route to Product Instructions for Arctic Silver 5
This might help as well. It has all different ways of applying goop based on ur heatsink type.
http://www.techenclave.com/cpu-mobo-corner/thermal-paste-application-method-119409.html
I usually apply thermal paste everytime i clean my cooler completely and that is like once in 6 months. U can even use it for years and without any problem.
It is made to stand extreme conditions so don’t worry. U can apply and forget or use whenever u wish to.