Pretty much any time you want.
"Four-Wheel Full Time" is essentially "All-Wheel Drive" - right down to the 50/50 torque split. Just as an experiment, I left the 89 w/242 (and a daily commuter) in 4FT for a solid year, and no ill effects.
Technically, you can use 4PT anytime there's likely to be "slippage" between the two ends - rain, light snow, sand, or whatever. Leaving your transfer case in PT on the dry and solid is likely to stretch and fail your drive chain - at the very least.
With 4FT, this isn't a problem - since it's a 50/50 split (rather than 52/48, IIRC,) between the front and rear axles, there's no difference between the output shafts to cause chain bind and stretch.
Granted, I'd not leave the tcase in 4FT for too long (the yearlong experiment was just that - an experiment. I did lose about 3mpg doing it...) because the extra drive causes some parasitic loss to fuel mileage. However, it shouldn't harm anything other than your wallet.
5-90