Faced with the prospect of giving a 90 minute talk, the QVNTS invitee will often adopt one of the following stances (or a combination theoreof):

A. "After my customary 60 minute lecture concluding with a statement of the main result and a sketch of its proof in a special case, I will now, thanks to the extra 30 minutes, be able to present my proof in all its details, paying fond attention to what happens when p=2 and my spectral sequence fails to degenerate after the 17-th stage."

B. "I've heard this seminar audience is large and takes an active interest in the lectures, asking many questions of the speaker along the way. So I'll plan essentially for a 60-70 minute talk, delivered at a more leisurely pace. Like this I will not be nervous about finishing on time if the audience peppers me with questions and comments."

C. "I will use the extra 30 minutes at the beginning to present the big picture which provides the background and motivation for the mathematics that I've done. This will also be helpful to the students and non-experts in attendance." (There are bound to be many of those: the QVNTS is a number theory seminar for generalists, the interests of its regular participants ranging from p-adic Hodge theory to additive combinatorics.)

Needless to say, strategies B and C are widely preferred to A!! As a general rule, you should strive to make the first 30 minutes (at least!) of your lecture accessible to the proverbial audience of graduate students with a bit of Cassels-Frolich, Davenport, Silverman and Hartshorne behind their belts, and not much else...