Paulo,
There is no set time. A lot depends on how the figure is selling, and whether Andy has ideas for a similar piece that will upgrade the set to be retired. Andy is fond of round numbers, and usually orders sets produced in increments of 500. I believe most sets today have production runs of 1,000. A really popular piece like the second Wittman Tiger, or the 88mm gun was probably produced to 1,500. Pieces that don't sell well are not produced beyond the original run of 500 or less. And then of course, there are the limited edition sets, which Andy has produced to 200 sets (1991 Gulf War Marines) 350 sets (Napoleonics) or 500 sets (Arnhem, the Axis leaders, the new Stug). When Andy was first experimenting in Polystone, and with the Winter stuff, there were very small runs of as little as 10 sets (the Winter Sherman, the Winter T34), 100 sets (the Winter Tiger, Winter StuG III, Winter Kubelwagon) or 200 sets (First Wittman Tiger, StuG III, Panther, Ambush Panther). For most of todays sets the average would be 1000 in a production run. I hope this answers your question.
Warmest regards,
Louis