Taxi's tend to be easier on oil than your average motorist, so you could maybe stretch it a bit, but then you have to guess what a reasonable stretch would be.

You could get oil analysis done. That'll cost more than an oil change, but if you can establish a basis for an OCI stretch it MIGHT save money long term.

Failing that, if there are "normal" and "severe service" recommendations, you probably don't need the "severe service " one.

In other words, should oil change depend mainly on time span or number of miles?

I think you'll find your handbook will state: "whichever comes first".

Yeh, but he's seeking The Truth.

OP, it depends mostly on number of miles, but it depends also on how those miles were acquired. Short trips, not fully warmed up (which isn't likely to be your operating pattern) are worst.

This is too complicated to get into in the handbook,.

If your car has an oil life monitoring system, its algorithm will probably incorporate usage patterns, so you could follow that.