I personaly would start from the "Spec". What should be the minimum delay, max delay, is it variable delay or fixed delay, should it be differential or not?
What size are your I/Os or is it just block without any output spec? (VOH, VOL etc)
You should probably make a calculation of the "stage" delay ( I am sure you had it during the course) and based on the calculation and the spec. you should be able to figure that out.
If you want just to put something quick together I would say 5/0.35 seems quite reasonable .... (though a little big)