It's a 60 day window for another employer to file. So you need to have another job lined up with the new employer submitting paperwork within 60 days.
[edit] Actually as of 2017, there's no time limit to apply for recapture. So if you came to the US on an H-1B, worked for 1 year, then you can apply for a new cap-exempt H-1B for the balance of the 5 years remaining at any time in the future. [1]
[1] https://www.federalregister.gov/documents/2016/11/18/2016-27...