Completion time could be one measure. Completion not being the proper term, beating time would be better.
Some games have a very short beating time once you know how to beat them.
Other games have also a very short learning time.
Some of these games might have an extremelly lengthy lifetime though as they beget to be played and replayed.
The current trend, though, is products with high, very high time to beat as they are filled with content, that most of the time, is unsupported by a proper gameplay.
One example already given: ME2 took a bit less than 30 hours to beat the first time.
All subsequent completions were achieved in less than 20 hours (including watching the cinematics)
When compared to games from the mid 1980s: they also took 30 hours to be beaten first. Once it is was done, subsequent completions (including these years) take less than 3 hours.
The difference: the content. 1980s games had three hours content, extended to 30 hours by the gameplay inventivity they showed.
2010 products have 20 hours (at least) content, extented by a few hours through trial and miss based gameplay.
A product like Wasteland 2 have fourty, fifty hours of content supported by whitewashed gameplay. People who will go for a second, third, fourth run know they are going to sink fifty hours in each of them etc
Time fillers, time sinks etc This is what those products are meant to be when assessed in terms of length.
Lengthy products are those products that would require dozens of hours to be finished, even when knowing it all.
