We file onward flight plans during the coffee break whilst refuelling
I file in advance, usually planning to depart 1h after landing if refuelling and 30 minutes if not.
When the filed FPL has a very long dogleg on the 1st leg (e.g. 200%, LFPN Toussus → LFOP Rouen), I first file the second leg (e.g. LFOP → EGSX), for the time I expect to leave the technical stop airport (LFOP). I then file the first leg.
You can’t do the opposite, as the EOBT for 2nd flight would be before the estimated arrival of the 1st flight (and Autorouter will reject the plan).
My experience is that IFR+IFR, it will reject if you submit a FPL whose EOBT falls during “another flight” (e.g. the previous flight EOBT + EET)
It will not reject if you file a FPL whose arrival falls during another flight (after the EOBT of the next flight)
My night instructor who is also IRI told me to leave 20 minutes between landing and the following departure, even when we only planned to do a touch and go. It is useless for VFR but I think he got this from IFR.