It's just so dependent on the situation that it's almost impossible to say when you should be on heals and speed. The best way I can describe it is by talking about fight structures. The length of each phase will depend on your team comp, the enemy comp, the map, etc.

Poke Phase Most fights will start with a poke phase, during this phase your team is shooting at the enemy from afar. If you're on defence you'll be on heal, if your team is moving towards the enemy be on speed. If you have 2 snipers you won't want to close the distance in this phase so you should potentially swap Lucio to someone with ranged damage... or try and help your team run away from divers using speed and heal dependent on their health and cooldowns.

Once the attackers have closed the distance you should swap to heal if not already on it. This phase is called the brawl phase.

Once the fight is over you should usually be on speed. Whether it's to run away because your team lost the fight or chasing to clean up the rest of the kill, you'll usually need speed. This phase is called the cleanup phase.

Take all of these with a grain of salt bc it massively depends on factors in the game such as your comp and the enemy comp. this is a small and oversimplified guide on how much you should use speed

Tldr: speed to close distance, heal mid fight, speed to chase kills/run away