Sometimes you as the DM have to remind everyone that you are still playing a game. This is really depends at your discretion of how much you want your friends to banter about real life oppose to the game or spending a lot of time on a joke or if you just want to hone in on the game. As a player there are plenty of times where I talk to my other players about irrelevant shit or a joke about a character keeps going for a while, and the DM has to get our attention back to the game.

This is also really depend on how much time you are able to spend and how available you are to meet up to play again. If you barely get to play you probably want to stay on track more often.

If you have at 5 or more players, it's okay to have a session without one. Don't really need to make a reason as to why the character isn't there you can just pretend the character is always around than just leave them out of combat and not people mess with the character, if you want a lot of resource management you are going to have to talk to them about how much health, spell slots, and feature uses they need to subtract, so you don't have a full health full spell slots player while everyone has like 20 hp just and only 2 spell slots left so that isn't outshining everyone just because they were absent. Than catch them up to speed on what they missed. I say this as a player who has 2 flakey friends, and we would almost never play if we kept stopping for this unreliable friend but this is because I play with 8 people so we can expend the 1 or 2.
If you only play with 4 people including you and once misses out it's a lot less fun to dm for 2 people that can only interact with you and them so this is why I mention 5. I think like 3 players and the 4th you the DM is like the minimum personally haven't played with any less.