I took them weekly when they were tiny, even when managing two at a time was challenging! They started lessons at 2.5-3yrs and are all still doing regular lessons/training sessions now (oldest is 11).
The cut-off in my head is end of primary, it’s great exercise and by then they will be extremely competent swimmers and technique will hopefully be ingrained enough that they don’t lose it.
As well as their training/lessons, we still try to take them weekly, it makes a huge difference to their progress when we do.