Per 47 CFR 52.15 (g) (4) (iii), “All service providers shall maintain no more than a six-month inventory of telephone numbers in each rate center or service area in which it provides telecommunications service.” 

If you do not anticipate using the telephone numbers in the next six months, any thousands-block  that is ten percent or less contaminated in your inventory should be donated to the pool.

For additional information regarding donations/disconnects,  see Section 11.1.1 of the (ATIS-0300119, Thousands-Block (NPA-NXX-X) & Central Office Code (NPA-NXX) Administration Guidelines (TBCOCAG).