I have removed the plenum in order to replace a new gasket.
I am wondering when exactly can/should I drive after replacing the gasket !

Couple of hours, a day, 3 days ???

Since I have replaced my injectors I will be needing to run the car on idle in order for ECM to learn the idle procedure so can/should I run the car on idle as well or should I wait for couple of hours, a day, 3 days ???

 

Give the RTV on the ends of the block a couple of hours to set before you start the engine... It will have skinned by then and the heat of the engine will set it completely. That is, if you went all the way down to the block and bare heads...

 

I just removed the upper plenum and TB so for the gasket couple of hours are good enough for RTV to seal ?

 

If you just replaced the upper plenum gaskets and TB gasket, you shouldn't have needed RTV, but if that's what you used instead of a regular gasket, yes, you should still wait a couple hours or you'll end up with vacuum leaks.