The best answer I can give is "it depends, but I usually kill the pet every fight"
If there's plenty of animals around, I usually just charm-tank equal-level mobs without buffing, break charm when they're both almost dead, then nuke them both down. There's less chance of a charm break if your pet is only alive for 1 fight, you get full xp for 2 kills per fight, and you have plenty of reserve mana if things go south. You can make things go faster by buffing your pet and dot/nuking the target, but say goodbye to that reserve mana, and it can be a bit harder to keep health levels even.
Keeping the pet alive drastically lowers your xp gain, IMO. You're either losing half the xp from each kill (and only getting 1 kill per fight) or you're blowing a ton of mana to out-damage your pet.
That said, there are times when I keep the pet alive as long as possible... If I'm killing higher-level animals than I can efficiently charm, I'll fear-kite with a lower-level pet. If my supply of animals is limited, I'll buff/heal the pet and dot/nuke the target (same if I'm killing higher-level mobs I can't fear).
For your stated preference of sitting with near-full mana killing things, the first option is probably the best. Pull 2 mobs, sit, break charm, nuke twice, repeat.
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