The primary use case of TL FSN is to purchase tickets for Fusion's ticketed Proof of Stake consensus. To purchase such tickets, the TL must be larger than 30 days into the future and thus time slices of FSN that span less than 30 days into the future can't be used for this purpose, unless they can be merged with other slices to make them big enough of course. In most cases, however, it means that only time slices that are bigger than 30 days will be able to generate any FSN through staking. Ideally you want a slice to be quite a bit bigger (perhaps 4 months), so that you can derive at least 75% of the value from it (3 months of staking, and one month of inability to purchase tickets).