Just got back from the San Blas (name most cruisers use). Awesome cruising ground. Easy check-in at Porvenir (already had Panamanian Cruising Permit).
Duration of stay not a big deal. We were there for about a month...many other cruising boats there for much longer. You can stay as long as you have a valid cruising permit
(1 year in Panama).
Get a copy of Eric Bauhaus' excellent cruising guide.
Already planning to return for a longer stay next season.