If I remember correctly I think it came on from 11:00 to 11:30 AM
That's my recollection also. Safely after the "essential" cartoons like Bugs Bunny were over.
Looking back, it's amazing that something so intelligent was allowed on Saturday morning. After all, this was the era of Super Friends.
There was other programming out there in the Seventies, like the Children's Television Workshop stuff, that didn't talk down to kids but elevated their minds instead, so maybe I shouldn't be too surprised. But Star Trek was always adult-quality fare, even things like the Power Records (at least when Alan Dean Foster was holding the pen).
In a way, the Power Records were even more influential in the pre-VCR era, because kids could listen to them again and again and enrich their vocabulary. (I credit "In Vino Veritas" alone with teaching me the words envoy, hegemony, palatable, subaltern, apportionment, protocol, and subterfuge.) But that's turning into another topic!