Gate controlled turn-on time
Too short a trigger pulse can cause hot spots to form in the thyristor structure and can damage the part. 

I suggest some tens of microseconds minimum is a good choice for modest thyristors (up to 40A). Gate dissipation will be negligible. 

A more limiting factor may be that if there is inductance in the circuit the thyristor current may not have time to build up to the holding current during the trigger pulse. In such a case you may not get the thyristor to conduct for the entire half-cycle of the mains- it will just briefly turn on during part of the trigger pulse then turn off again after.