It's just as long as the last outgoing HTLC expiry, and since I'm lazy, I keep it at 10,000hrs. This is specifically helpful if you have FCs with pending HTLCs, since each of them needs to go onchain. If those are mostly outgoing, I stay at 10,000hrs and switch to 10 minutes, restart LND when the fees are low are all pending HTLCs are mature. In a result, those pending HTLCs get batched into one tx, hence saving you onchain costs.

Then back to 10,000hrs.