How long do you hold lots before building starts?

[Roy Oliphant]

Ok, this is a newbie question but here goes:

Do builders carry an inventory of vacant lots and how long do they hold them before building starts?  If not, who do they get the lots from when they are building spec houses?

I come across a number of below market value lots while looking for other opportunities and am wondering who would be the most interested in them.

Thanks!

Roy