I think the rule of thumb is they should protrude by a length equal to the diameter. Certainly no less.
(Which, oddly enuff is the same as for doing home made riveting with a ball pein hammer.)
The Other factor is whether you want a flush finish on the peined end.
See the link below which shows imperial grip and pein data, I'm sure this will help. http://www.rivetsinstock.com/rivet56.htm