In last 3 years I used zero. I start my sun sessions in March and by May/June I'm pretanned. This allows me to handle Polish sun without SPF in all summer.

The only time I was close to using SPF was when Spanish Costa Del Sol sun got to me after 4 days. But *only* on the back, which I usually am not tanning in my spring sessions. I refrained from it though and picked up a special UV tshirt for snorkeling. No chemicals, just clothes.

I think if I tanned my back in the spring I might have gotten away with that.