I think I can say something about Fischer time.
First, some estimate about the game time of »1 min, then 3×10 s japanese byoyomi«: player will use on average less than 10 s per move, I’m guessing maybe two thirds, so 6—7 s? At 300 moves, that’s about 33 min, plus the two minutes of main time, so 35 min.
The Fischer time bonus part can be a bit faster due to the much greater flexibility, I’d say 5 s keeps with the spirit. That’s 25 min for 300 moves, which leaves us with 5 min basic time per player.
So, my recommendation minimum would be »Fischer time 5 min basic time plus 5 s per move« (often abbreviated as »Fischer 5/5«).
Technical note: this assumes no cap (“up to…”). If you have a cap, that disturbs the calculation above, the game takes longer (because players are incentivized to play slower when near and above the cap), and the players then also have more to think about the clock. Since OGS sadly refuses to remove the mandatory cap, at least set it to the maximum.