Sorta. To oversimplify, the enzymes break long starch chains into smaller sugar chains. As soon as the starch is split, it is converted. Initially, enzymatic action results in dextrins - complex sugars that are unfermentable (in finished beer, dextrins are responsible for body/mouthfeel). Additional enzymatic activity must further deconstruct the molecule chains before the sugars are simple enough to be metabolized by yeast. Therefore, conversion can be complete in a very short period of time (15-20 mins), however, the desired results may not be achieved until after an additional period of time. Too long of a mash may result in less body and/or more dryness in the final product.
Does that mean a two hour mash is too long? It depends. For a crisp, dry Czech pils where some of the grain bill is slightly under-modified malt, a two hour mash may be just fine (or even required. For a Belgian wit, a long mash may destroy some of the desirable velvety mouthfeel.
Of course, all of this is also quite temperature dependent. To retain some body after a long, unattended mash, start the mash hot (in the upper 150s or even low 160s). To achieve a very dry beer, start the mash at a cooler temperature (in the low 150s or even high 140s).
Read here for a bit more in-depth explanation of enzymatic activity in the mash:
http://www.winning-homebrew.com/enzymes.html
