It depends on the conditions — and that's true of any gel pack. As a general guide tested under normal indoor conditions (around 20–22°C):
Cold: The gel pack stays therapeutically cold for approximately 2 to 3 hours.
Heat: The gel pack delivers effective heat therapy for approximately 60 to 90 minutes after microwaving.
Once applied to your body, the pack gets to work immediately — transferring cold or heat directly to where you need it. That energy transfer is exactly what makes it effective, so the active session time is shorter than the total retention time. For most people, a 15 to 20 minute session is all you need. Refreeze or reheat between sessions and it's ready to go again.
A few things that affect performance: ambient temperature, how long it's been out of the freezer before you apply it, and whether it's stored in the travel case between uses. On a hot day or in a warm room, expect the lower end of those ranges.
Tip: Keep the gel pack in the freezer between sessions so it's always ready. If you're heading out, store it in the CompressiveX Travel Case — it significantly extends cold retention on the go.