Is 90 minutes enough time to see the gardens?

You can take more time if you'd like, but 90 minutes is plenty of time to walk through the gardens. For me, I decided to get a membership to support this green space. So it's nice to go often and walk for 45_60 minutes sitting as I find a spot to admire the pond, a Grove or a nice planted area.

I'd say so if you just want to walk through it. You can easily spend more time but it terms of walking it's 2-3 miles total.

Depends on what you are going for. If you just want to walk through and see everything, 90 minutes will be enough time if you walk briskly through the gardens. However to really enjoy Descanso, I would recommend sitting by the lake and packing a few sandwiches, some drinks and just listening to the faint heavenly sounds coming from the amphitheater. That's 90 minutes we'll spent.

Allow yourself at least 3 hours to enjoy the beauty of Descanso at a leisurely pace.

It is enough time to see the majority of the gardens. There are however many paths and loops to go hiking around.