A good time to have it checked is when the engine oil is checked. See your Allison Automatic Transmission Operator's Manual to find out when to change your transmission fluid and filters.
The Allison Automatic Transmission Operator's Manual that came with the vehicle shows how to check the automatic transmission fluid and what fluid to use.
Your automatic transmission filter requires periodic replacement.
Consult the Allison Automatic Transmission Operator's Manual that came with the vehicle for proper change intervals.