When is the best time to buy tickets

I will be in New York for a couple of weeks in April and wanted to see a number of plays. I was wondering when is it better to buy tickets early, like now, and when is it better to wait? For instance, I bought a ticket for Talley's Folly quite early and got a very good seat, much better than i could get now. But I am holding off buying tickets for The Big Knife because the seats currently on offer aren't that great, and it was pointed out to me on another thread that some seats are held for subscribers and may be released at a later date. I ask because because I just bought tickets for The Mound Builders and there were very few seats left on the days I could go. I wasn't sure if this was because, like with the Big Knife, seats were held for subscribers, or they were selling quickly because the tickets are only $25. Also, there wasn't a pattern to the seats that were unavailable, as there was with the Big Knife (i.e, in the centre.) Was it best to buy early or did I fall into a classic tourist trap? (not that I mind too much as the theatre is quite small).

Also, and apologies for the long thread. I wanted to see Cat on a Hot Tin Roof, but I arrive late in the evening on the day of its final performance. Is there any chance this extends. Or am I more likely to see Scarlett Johansson running through JFK to catch a flight to the shoot for a new comic book movie than I am to see her on stage in Williams' play?

Most shows release unsold house seats and premium seats (at the regular non-premium top ticket price) 48 hours before the performance. Check Telecharge or Ticketmaster at 6:00 a.m. on the day before the performance you want to attend, you will probably get seats even to SRO shows (okay, maybe not THE BOOK OF MORMON).