I don't think there's any predetermined timeframe for unlimited listings (or even if eBay technically grant that still, although some of the limits they grant would be as good as), it's mostly based on selling history - a lot of older accounts won't have ever had any listing limits placed on them, so in that way it was kind of luck of the draw (all new accounts should have them, and I think also older accounts that weren't registered for selling until after the limits were introduced). 

The best you can do is continue to request higher selling limits and hope eBay will grant them, which I believe you can do once per 30 days.