I saw a reference to this recently and took pause. As I understand it, mash temp and the effects it has on the various enzymes effect fermentability (that is, the amount of fermentable vs non-fermentable sugars), not mash time.
The real answer is that it depends, but it certainly can.
The things to consider:
- Temperature affects the rate of enzymatic reaction - higher temperatures will mean the reaction happens faster, so high temperature mashes reach their limits of fermentability faster (i.e. mashing at 154°F you are not likely to see much benefit from extending your rest from 60 to 90 minutes, whereas if you're mashing at 148°F, you may well)
- Denaturaion of enzymes - probably the most crucial point to consider. The higher the temperature the faster certain enzymes will denature, especially beta-amylase (which will survive about half an hour at about 152°F). So if you mash on the high side, the beta-amylase will have been denatured well before the typical hour-long rest is over and all continuing enzymatic activity (i.e. increased fermentability) will be due to alpha-amylase, which is capable of producing fermentable extract, but is not ideal for it. Likewise, mashing low (~146-148°F) the beta-amylase will survive much longer and should be able to continue to create fermentable extract
- Changes in substrate concentration - as the mash continues and starch and dextrins are broken down, the enzymes have fewer molecules to work on and the rate naturally slows. Beta-amylase specifically has a higher affinity for larger molecules. When you're relying on alpha-amylase late in mashing to create fermentable extract, remember it only has a very limited number of substrate sites on which it can act and so while it can continue producing fermentable extract, it will be at an increasingly decreasing rate.
Basically it's more about how much time you want to spend on a decreasing rate of return of fermentability. Any mashing process has a natural limit to the fermentability it can achieve, and the closer you get to this limit the slower you'll be approaching it, but in many cases extending the mash can and will produce increases (albeit probably very small) in fermentability. The reality, though, is that most of the time it's just not worth it to wait an extra 30-60 minutes to get a tiny extra bit of fermentability.
Yes. You are correct that mash temperature is what primarily affects the fermentability of the wort, but time is also a factor. Modern brew malts convert the majority of their starches to sugar in as little as 10 minutes, but beta-amylase takes somewhat longer to work than alpha-amylase. That's why some recipes recommend a 90 minute mash for lower mash temps. I've done overnight mashes and those do dry out pretty well, but the yield for time is very low after 90 minutes. Fermentability is also affected by the grist composition, mash thickness, and ph. The attenuation limits of the yeast can also limit fermentability.
For further reading, here's a BYO article