The following to sentenses have different meanings
Er schätzt sie.
Er schätzt sie ab.
But looking at
Er schätzt die zurückgelegte Strecke auf 5 Kilometer.
and
Er schätzt, dass er zur Lösung der Aufgabe ca. 3 Tage braucht.
it seems, that schätzen is used in the sense of abschätzen, without using the ab explicitly.
I admit, the more I think about it, the more I get confused.
Sorry I can't translate the example sentences. Just started arguing with my online translators, which seem as confused as I'm.
Are here some rules to understand the proper usage?