Is the fighter generally pious? Is divine intervention the only option? Will the character die without it? Is the Fighter giving up a turn in a dangerous combat for this (semi-literally) hail mary play? Does the God favor their cause?
I might let the DC get down to 20 if all the factors lined up, particularly if the situation was dire. Generally it would range from higher to impossible. I don't think the DC would ever be below 20, unless maybe the characters were on a mission for that God or in a place sacred to them, or there was some other comparable factor that both made it a cool story moment and an unrepeatable one.
Basically I think this would be really cool to do once, but really shouldn't be a regular part of the game. So I would be aiming for a DC that has a reasonable chance of success, but still has to be high so it feels like a just-got-by-by-the-skin-of-their-teeth situation, so that they know not to try it regularly, or else there has to be an obvious "this only works because you stand within the remote sacred grove" sort of limiting factor.