I frequently mash for 3 or 4 hours. But sometimes I'll just go for 1 to 1.5. I haven't noticed a huge difference in fermentability between the two times for a given mash temp. (I do see maybe a 1-2% improvement in efficiency though.) If you want some added insurance against over attenuation, you could bump it a degree, maybe two tops.
My observation fits with a graph I saw in a brewing book someplace. It showed beta amylase activity versus time at various temperatures. IIRC, at 150 F and higher, it is virtually gone after 45 minutes, so there's not much hanging around in your mash in hours 2 and 3 anyway.