In O scale, about 4 years. They've done 11 pre-built structures so far. Three buildings were released in each of the first 3 years, only 2 last year (Dunno why....that's all they did.)
Of course, they've been doing HO...and N...pre-built buildings a lot longer. We (the O crowd) look forward to their annual releases. And that depends on how well the latest releases sell....hint, hint!
Now, with the addition of their lighting system, they've really set a high benchmark for the structural part of scenery. For all of which we surely say...'Thank You, Woodland Scenics!!'
KD