I doubt it is so much possible in triangle, but, if to try, then its like 10 hours flight, launch 9:00LCL, 9:30LCL J.Gora - S.Wola - Plock- J.Gora or S.Wola-J.Gora-Plock-S.Wola.
At least on simulation there are quite often early thermal starts in Poland and some cloudstreets should give tomorrow some lift under cloudbase up to 1930LCL, if last leg is adjusted with streets and not descending from cloudbase.
So it is average 100km/h flight with very good glider.
For out and return flights there are start options 1030LCL and lasting also to 1900 . So it would be 8 hours under street back and forth with some good luck, using very early and weak climbs.
The thermal day would be average and with inversions, but just the start and end are long for many places.
