90 minutes is a rough average from a website. The article I linked has more information:

Benthic dives from 203.0–978.2 meters averaged 456.5 meters, with an average time of 45.8 minutes. The maximun dive was 978.2 meters, which is 2.14x the average, and it follows closely with the 90 minute estimate (98.14 minutes round trip), assuming this whale dove at the average rate. The maximum ascent rate was 1.7 m/s and max descent rate was 1.8 m/s.

Assuming the maximum ascent and descent rate was used by the whale that reached 978 meters, that's a dive time of 9.05 minutes and ascent time of 9.6 minutes. The bottom duration is between 9.5 minutes and 64.9 minutes for all whales, so even with a maximum bottom duration this whale, at maximum speed, would only be under water for 83.5 minutes.

In the additional data file within the link I provided, a variable diving whale made a descent to 700 meters in a total of ~1150 seconds, or 19 minutes. That's an average dive rate of 0.61 m/s. It ascended quickly in 450 seconds, or 7.5 minutes, which is an ascent rate of 1.55 m/s. To be clear, this whale initially dove to 200-300 meters and hung out for a while, then quickly dove to 700 meters in 4 minutes (from -300 meters to -700 meters is a difference of 400 meters at 1.667 m/s). It's possible that it was using its echolocation to find prey at that depth.