
Isaac Zephyr |

I'd probably say 2 or 3 combat encounters.
One against a lot of enemies, maybe six. One against a good amount and a leader, and then a big boss fight, something that'll sponge some good hits.
Granted, my experience with Starfinder so far has been in our 3 hour game time we normally only get one combat done. Granted we have a good amount of in-between and we're only level 1, so other than our soldier it's usually two turns for us all to get our weapons out before combat can really begin. Also level 1 most of us do about 1d4 damage when we hit, which let's face it, we miss a lot of shots.

Kvetchus |
It depends entirely upon your group and how efficiently the games progress. My group has, in the past, spent a solid hour fighting one small group of goblins merely because one guy wanted to get all tricky with his drone and ended up eating tons of time farting around. This is especially true if you have to look up and interpret rules around something you haven't done before.
Other times, the group isn't doing anything unusual, no one is getting distracted by anything, and we plow through a huge fight in 4 rounds... bing bam boom.
You need to take into consideration how your players actually play and how complex/large the fights themselves are. It's really hard to give an estimate because a slow group, or even worse, one with a player that insists on looking up the rules for everything and (even WORSE) arguing with the GM over them (despite the fact that the GM *is* the rules)... those things will make 1 combat take hours and hours.
For my group - I find 3 or 4 smaller-scale combats OR 2 smaller scale combats plus a few skill test type things (2-person hacks while someone else has to do some engineering, and the Envoy is keeping the security center calling on comms to check-in busy.... bam, boring conversation anyway, we're gonna have company!) plus a larger climax type fight can usually work out in one sitting, unless a particular player happens to be there, in which case.... maybe just skip to the end :)