This is rather long time (you can make a commercial app in this interval). I think it's ok as long as you don't have to provide source code Smile
Otherwise something like a month is more like it...
I don't think it is. Look at the 4e4 compo. Participants had 5 months to work on a entry. Still, out of 43 entries, I think only half of those really were finished.

Although 4 months is a long time. The endresults will be better.