Do you want it to always be up, or just while in combat? Either way you want the FlaskHelper plugin.
If you want the flask buff to always be up, you just need to set a condition that's always true, like "me.HealthPercent < 101" or something (just putting "true" doesn't work, presumably because of how the conditions are parsed). The plugin checks whether you already have the buff and doesn't use the flask if you do, so it won't just hit it 3 times in a second and waste all the charges.
If you want it to always be up while enemies are nearby, you want "FH.NumberOfMobsNear(me, 50) > 0", where 50 is the range (roughly the whole screen) and 0 is the number of mobs that can be nearby without using it - set it to 1 and it'll be used with 2+ mobs nearby, etc.
