It takes about 1 second to change to Ifrit when Dante is using Alastor. (Without going into the weapon menu.)

Hmm, Dante can first build up 3 symbols, and never let it go below 3 symbols. He can build it up to 8, 10, whatever, go to Devil Trigger, and stop using Devil Trigger as soon as his gauge hits the third symbols so that he will be safe for Auron's Overdrive. He can stop Devil Trigger anytime he wants to, it doesn't have to burn out to the end.

Like I have mentioned, Auron isn't that good in ranged battle. The duel is held in a huge open area, so the advantage has clearly gone to Dante's side. Dante can always keep a good safe distance between Auron and at the same time deals moderate damage to him.

I forgot to mention, when Dante is in Devil Mode, his health will regenerate in a moderately fast pace. (Moderately fast in the Normal mode of the game.) His attack damage and defense will be significantly higher, and if he's using Alastor, his movement will be twice as fast.