If you're a regular member, as in NOT Premier Silver or have a RZ MasterCard, then the maximum threshold you can have is 1,000 points (a $20 certificate). If you choose that threshold and have anything less than 1,000 points then the points don't expire until February (see below*).
A member who has a RZ MasterCard can have a threshold of 25,000 points (certificates that add up to $500, since you cannot get a certificate issued for more than $95 now). A Premier Silver member can choose to "bank" their points and these points do not ever expire as long as the member retains the Premier Silver status.
*All member's points that are not "banked" (Permier Silver) issue into certificates every February and any points 249 and below (250 = a $5 certificate) are forfeited/canceled.
Before posting I research everything first or use my own experiences. All my posts are not representative of Best Buy, its affiliates, subsidies, parent if any, stockholders or anything to do with Best Buy. I love Best Buy even though I work for a competitor, I do not now nor have I ever worked for Best Buy.