Hi Beaud.
Ambient temperature as well as tub insulation have governing factors.
I'm assuming that Texas is warm (it always looks it in the movies)
Here in the UK, our summer temperatures are about 25 deg C.
My tub which has an insulated cover and insulated foam internally, loses about 1 degree over night.
Night time temps are dropping to sub 5 degrees C, and I'm losing at least 2 degrees over night.
In the summer, it would increase about 1 degree per hour.
At this time of year its taking about 8 hours to increase by 6 degrees.
So as you can see, there are lots of permutations to consider.