ideally forever but it will vary & most seep a little. I bought an alum stat housing and drilled it with a .339 "R" drill bit then tapped for 1/8 NPT then screwed in a 1/8 NPT/schrader brass tank valve that sells at O'reillys for less than $3 then pump it up with your air chuck. took minutes from start to finish. Far cheaper than a Stant radiator pump up universal kit tho the kit has the adapter to check the cap also. Holler how it goes. EDIT Drill a miniscule hole in the stat flat ~1/32 or just use your smallest bit (wont affect cooling at all & vastly eases initial fillup time. MORE EDIT monitor your time as you'll hit 15 in seconds! EVEN MORE EDIT stat housing was pot metal or cast iron, just felt like alum when I was working with it