A clue on what to use preferably may give you the answer to the question when you ask:
Question: "Was schätzen Sie, wie viele Stunden das Projekt benötigt?"
Answer: "[Ich schätze,] wir brauchen ungefähr 10 bis 12 Stunden"
This is when you ask for a rough estimate of the time needed for the project. In both cases "abschätzen" is wrong and can not be used.
In another variant you may ask:
Question: "Können Sie schon abschätzen, wie lange Sie brauchen?"
Answer: "[Ich schätze,] wir brauchen noch 10 bis 12 Stunden"
This is when asking for a more precise estimate of the time needed. This variant is used less frequently. In this case you can answer the question the same way but still you can not use "abschätzen".
In both answers "schätzen" can be omitted but it may be more expected when you do so in the second case.
In most cases "schätzen" is to be used preferably. This helps to avoid errors from using "abschätzen" in the wrong place, e.g. when building an adjective
"In geschätzten 3 Stunden sind wir fertig."
This is correct - "abgeschätzt" can not be used here.