Probably not until the mid-20's. Don't know, it's never come on for me, and I've had my tires down to 30 psi. At 30 you can feel a bit of squishy-ness but it doesn't feel dangerous.
It would be a pain for those who air down their tires when offroad if it started dinging at that point. 20-25 or even lower if you have beadlocks is great for offroad.

 

OK folks. Thanks. I have settled on 31 for a low pressure alert on the spare. That is 35 minus 10% rounded down. FWIW, the spare had 40.5 psi from the factory.

 

OK, I must have missed this somewhere: where is the threshold setting for the alarm in FORScan? I just switched from p-metric to LT tires and expect to be running higher pressure as a result.

EDIT: Found it in the spreadsheet.

726-40-01 xxxx FFRR xxxx — FF is the front pressure (in hexadecimal), RR is the rear pressure (in hexadecimal)