For those of you who can't wait to try the new gargoyle stoneplate in practice mode.

The highest possible health acheiveable in game is somewhere around 4, 294,000,000, due to the type of storage integer used for health. 4,294,989,164 is the number I found, but could quite possibly be different.

When you surpass this number is when your health drops back to zero restarting the cycle again.

Source: http://forums.na.leagueoflegends.com/board/show-thread.php?t=1258408&page=15