Years ago I ran a lot of experiments on mash times. I found that indeed the limiting factor is fermentability/attenuation, NOT conversion efficiency. For my setup, a mash of 20-30 minutes was long enough for "good" attenuation (e.g., in the 70-80% range) about 50% of the time. The other 50% of the time, I was getting very high final gravities like in the 1.020s. 35 minutes was better. 40 minutes was always long enough.
Conclusion: I usually mash for 40 minutes these days, or 45 minutes if I'm lazy. I only mash for 60 minutes or more when I'm making a monster high gravity beer or when I want a really dry beer like a saison, or if using a very high percent (like for a Munich dunkel) of a very low enzymatic malt (e.g., Munich malt). Otherwise, for 95% of beer styles...
40 minutes is plenty. Save 20+ minutes of your life on every batch, if you want to. Works great for me.