Thumb pressure varies depending on what you are doing, but generally I put as little as is required. When doing scales for example, I put almost zero pressure on the thumb and use the thumb more as a guide for stability. In most cases as much as possible I try to put the pressure on my biceps and arms rather than using all the strength in the thumb to fret the strings. This will really prevent your thumb from fatiguing and getting strained, especially when doing barre chords.
"Success grants its rewards to a few, but is the dream of the multitudes.
Excellence is available to all, but is accepted only by a few." - Christopher Parkening