Is there a heuristic for how much time handicap to give players of different skill levels? I was curious if there was general guidelines for time handicap ala how there is for piece handicaps (e.g. I've read 1 pawn ≈ 300 elo points for GMs). Is there a way to compute a rough estimate of how much time less the better player should have in a game? For example when I play blitz with a friend who is better, we have even odds  when poor player myself (1200 lichess) gets 5 minutes and stronger player him (1900 lichess) gets 1 minute. Of course this depends on the player and is probably nonlinear but I was curious of a way to get a rough estimate of where to start the handicap before I begin the binary search for the best handicap value.
we have even odds when poor player myself (1200 lichess) gets 5 minutes and stronger player him (1900 lichess) gets 1 minute

That's probably not too far out. As I wrote in this answer about some of the winding down at the prize-giving party after the 2019 Isle of Man:

IM Lawrence Trent (~2400) was playing blitz / bullet with Caruana with 3 minutes to Caruana's 1 minute and losing in a proportion of about 2 to 1. He then took on Nakamura with 5 minutes to Nakamura's 45 seconds with similar results.

Interesting differential there. At those time controls Caruana and Nakamura were playing bullet to Trent's blitz. Caruana is stronger at standard time controls than Nakamura but at bullet Nakamura is just a beast and you have to take that into account and not use standard ratings.
It's worth adding that you can't play with an increment either in these circumstances. A much stronger player can almost play just on increment alone.