Since the duration is not specified it is up to the GM.
However, I'd focus on two points (emphasis mine):
When you cast the spell, you change the current weather conditions,
which are determined by the DM based on the climate and season. You
can change precipitation, temperature, and wind. It takes 1d4 × 10
minutes for the new conditions to take effect. Once they do so, you
can change the conditions again. When the spell ends, the weather
gradually returns to normal.
When you change the weather conditions, find a current condition on
the following tables and change its stage by one, up or down. When
changing the wind, you can change its direction.
Based on the above I understand two things:
- If the weather has been changed more than one time (e.g., warm -> cool -> cold) then when the effects are reversed the same order should be followed (i.e., cold -> cool -> warm), otherwise the change is not "gradual".
- The fastest effect can be achieved in 1 x 10 minutes. The slowest in 4 x 10 minutes. Thus, a gradual change of one stage should be anything between 10 and 40 minutes. Personally, I'd go for 20 minutes per stage but you can also ask the player to roll.