I'd say one per per person per month, and it has to be in the month (+ 1 week for the builds that finish late in the month) that it was finished in. No banking up entries for the next 4 months.
Obviously the honour system will have to come into play a bit here, but as long as there are no fabulous cash prizes and 'round the world cruises on the line nothing will be gained by gaming the system.
Maybe even require a build diary thread if you want to limit it further.