Infinity

Not entirely sure, but I remember seeing a few videos where a mage casted Infinity and it timed out exactly 66 seconds later. It may have been due to lag. I'll try to dig one up.

This is the way I thought it works:

 Spoiler

0-6: +0% damage
7-12: +10% damage
13-18: +20% damage
19-24: +30% damage
25-30: +40% damage
31-36: +50% damage
37-42: +60% damage
43-48: +70% damage
49-54: +80% damage
55-60: +90% damage
61-66: +100% damage

Which is effectively a 60 second buff, just delayed by 6 seconds.

Here's the first video I could find with Google:

Infinity is casted at 0:01, and the buff doesn't disappear from the top right corner of the screen until 1:06.
[video=youtube;_CHIHgCIvmc]http://www.youtube.com/watch?v=_CHIHgCIvmc&feature=related[/video]

Or there's another option yet:

The 40 second duration is increased to 60 seconds, but the 4 second interval remains. This would give you +150% damage.

I think this is unlikely. I haven't seen any increases in damage that match that large of a damage boost. In the above video, the highest non-crit hits at the beginning seem to be around 130k, and the highest non-crit hits at the end are around 260k.