You can hold out and cash them in all at once. The purchase just has to be made within 30 days of the last month you are getting money for. I just got an expense check from my work for $120 for 6 months(Jan-June, bought the parts in beginning of July) of commuting on a $200+ bill of parts. At the end of the year I'll be able to get another $120 or however I decide to split it up in between.