What do you mean by finish? I was taught by a brewer friend that lager takes a fraction of the time ales do. (due to faster fermentation from the yeast being on top and nearer oxygen).
Lagers are (or should be) fermented at around 10 deg C, and thus take longer to ferment than ales that are fermented at room temperature. They also need to be 'stored' (that's what 'lager' means) at 8-10 deg. for several weeks after primary fermentation to develop that characteristic lager flavour.
That is exactly backwards - ale yeasts are so-called "top-fermenting", prefer higher temperatures, and typically finish faster. Also, there is typically no oxygen available during fermentation beyond what is dissolved in the wort to begin with. Any oxygen in the fermenter's headspace is quickly pushed out by the yeast's CO2 production. The dissolved oxygen is quickly consumed by the yeast.
That said, I believe many breweries have figured out rapid lager production processes and are able to turn around lagers in about the same time as ales.
Common misconception. Fermentation happens throughout the wort, which is one reason you want it well-aerated before pitching the yeast. Yeast, when _done_ fermenting, flocculate at the top or bottom. Very little oxygen should be near the top of the wort during fermentation, due to CO2 accumulation.
