[Basicz]

Maybe try a timer.

pawn Код:

new 
Float: Health;
GetPlayerHealth ( playerid, Health );
if ( Health < 100 )
{
SetTimer ( "RefillHealth", 1000, 0 );
return 1;
}

// Add it in a blank line ( Not in callbacks I mean ).
forward RefillHealth ( );
public RefillHealth ( )
{
SetPlayerHealth ( playerid, 100.0 );
return 1;
}

Sorry if it will not work, because I didn't test it.