No deadline for reconsideration because it is an unofficial route to challenge the decision and the agent is not obligated to respond to your request (unlike JR). He will do it if he is "nice". However, the longer you wait, the higher chances that he will no longer re-open your file as he will move on to new files. So do it ASAP, few days to few weeks, not more.
In general, You can file reconsideration for as long as your status remains "in process" (sometimes they forget or deliberately keep it in process). Once it is "decision made", you have to rush it ASAP.