:state=0:state=1
:vargreater=fh $PH,plrhealthgreater=125:etimerstart,state=1
:vargreater=fh $PH,plrhealthless=126:etimerstart,state=3
:state=1,etimergreater=4000:etimerstart,state=2
:state=2,varless=fh $PH:plraddhealth=1
:state=3,etimergreater=5000:state=4
:state=4,varless=fh $PH:plraddhealth=1
:ALWAYS:setvar=fh $PH,subvar=fh 1
This will do what you want. Adjust the health values as you want. While your health is greater than 25%, your health will regenerate after 4 seconds of not being shot. While your health is less than 25%, there is a slightly longer delay before regenerating.
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