Hi,
It seems to be question to Joomla developers to improve that.
But I cannot belive that you didn't click SAVE button every 15 minutes like every smart developer does.
Tip: Set 60 min session in day when you plan working longer and click something in joomla admin area every 20min, this may help.
By now I cannot return your lost work, even if I want to. Sorry. Only maybe hosting company did backup in meantime, if yes - you're lucky.
probably there is also for J4 extra extension for session timer -- at least was for J3 - Session Keeper.