Depends on the library and the type of book. Typical time frames are 10 years, 7 years, 5 years, or 2 years. For example I work in an busy urban library in the children’s room and we were running out of space in our fiction novel section so we did a 5 year then a 2 year when the first one didn’t make enough space. Our biography section grows less quickly so we do 7 year weeds.