How much should i wait after re pastibg my laptop (Helios 300g3-571)

Hey Sezokiller,

I hope you are having a great day.
If I get your question correctly, you put new therman paste on your CPU or GPU or both. 
Usually you don not have to wait until you can use your Laptop again. If you think the paste must dry or anthing like that: This is not the case.
Time will not do any difference to the paste, just make sure, that no paste exacped from under the heat pipes to the restly computing parts like the battary or the ram. 

If you want to be very sure, you can wait for 4 hours or so, just to be sure that any possible amount of water evaporated inside the case. Usually, this is not the case though.