I guess it depends on the software itself. Say it's just some teensy little app with one use. A few minutes will probably tell you: yes! it's worth keeping. Or you'll probably have vaporized it right away.
Whereas some real hefter of full-blown app may take weeks of solid use to "warm to"
One evaluation we did with a large Scandinavian car manufacturer, it took them 18 months of solid use to make a decision.