Good morning Kim,

I give mine at least two minutes and only very rarely will it be any longer. So far I've had no issues whatsoever. I usually turn the amp on and while it's warming up on standby I use a pedal to check my tuning. Usually by the time I've finished it's had the two minutes. 

Some people will say you don't even need to wait that long yet others will advise waiting a good while longer. I've had no issues in almost two years.

Cheers,

Si.