Every summer seems too much to me. Every other year would be about right, assuming these movies are made in a normal way.
To do one every summer - and maintain quality - would surely mean:
- either a heavily preplanned story filmed in batches of 2-3 movies back-to-back/concurrently a la LOTR was
- or an anthology approach with varying casts whereby you could overlap schedules/actors with no problem.
Or, I suppose, a blend of the above whereby the "main" story films are every other year and the intervening years have "filler" films focusing on other characters/time periods. That might jive with the other reports we had a while ago about some films focusing on particular OT characters. I haven't been following the rumour mill to know if that's still the plan.