When you need to. I know that’s vague but no one out here can really tell you. But here is a starting point. First, check for pad wear. If the pads are worn too much or worn unevenly, replace them. Uneven wear indicates a problem between the caliper and pads. Then squeeze the brake pedal against the front nerf bar. The pedal should not touch the nerf and should not creep forward as you maintain pressure. Now bleed the brakes and test the pedal travel again. Use ONLY the brake fluid approved for your brake system. When the pedal starts to travel more or the pedal does not seen as firm, bleed again.
We come from road racing a Sn Rotax and I bleed ours before every race weekend and checked the pedal travel several times during the weekend.
Be safe and have fun.

A common problem… mushy brakes that is.  When you replaced the seals, did you do the caliper as well as the master cyl?

You may have a very tiny leak near a fitting on the brake lines.  Those are tough to find, and it doesn’t take very much air getting sucked-in to cause mushy/fading brakes.

Usually when your master cylinder starts to bypass internally, so does the caliper seals.

I always rebuild the whole system

And yes, it is very  possible to have a small leak or hairline crack at the line fittings, check   those too.

We always check the braking system, before we check our power plant, brakes should be the first thing to check before a track day, as a safety rule.

Always flush and bleed hydraulic brakes.