This is a challenging question, because it depends on the volume of swimming or number of sessions you do. But you have to remember that in a 750m swim or longer, 95% or more of it will be done using aerobic energy systems. As a result, the majority of your training should be at an aerobic level – i.e. always using/transporting oxygen to help transform glucose/fat into energy.
I’d suggest somewhere between 60 and 75% of your overall swim training. That said, aerobic doesn’t mean one-paced. Anything that’s below your lactate threshold (where lactate starts accumulating in larger quantities in your blood) is aerobic, from your warm-up and cool-down pace, all the way up to your CSS (critical swim speed – i.e. your estimated pace over 1.5km +).
The longer the swim you’re working towards, the longer your training volume will likely be (makes sense!), and so the more distance you’ll cover at an aerobic level. However, doing shorter and harder efforts will help increase your fitness – and ability to maintain form under duress – so don’t neglect speed and power work at the expense of just swimming longer.
Remember to maintain a good mix of efforts and intensities throughout the year. It’ll change depending on the season, but that balance will help you get the most out of your swims. It might be that in the winter and early spring, you do a higher percentage of your training aerobically, and then in the months leading up to your event(s), you do less aerobically and a little more above threshold and high intensities.