Akali's sustain and why it probably needs toning down
Back in Season 1, with pre-nerf Hextech Gunblade and Akali's Twin Disciplines, she gained a total of 47% spellvamp.
Hextech Gunblade nerfed: 
Graves Patch http://leagueoflegends.wikia.com/wiki/V1.0.0.127
Akali's Twin Disciplines nerfed: 
Fizz Patch http://leagueoflegends.wikia.com/wiki/V1.0.0.129
With the nerfs, this lowered Akali's total spellvamp to ~35%. So Akali initially was healing for about half the damage she dealt, down to about a third after Fizz patch.
The thing to note is that in Season 1, the environment for squishy melees to thrive in was much harshier. Resistances were much more stronger in Season 1, and being behind was much less forgiving than currently in Season 5.
The point is, since alot of systematic changes has happened since Season 1 and now it is Season 5, the strength of 35% spellvamp currently is NOT the same as the strength of 35% spellvamp in Season 1. Why?
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Resistances are overall weaker, therefore more damage dealt means more healing received from spellvamp.
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Penetration is also generally stronger now, and again, more damage dealt means more healing received from spellvamp.
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Akali herself got buffs since Season 1. Specifically this thread, http://boards.na.leagueoflegends.com/en/c/gameplay-balance/hMJmmtbp-a-really-simple-change-to-add-finesse-into-playing-akali.
Akali can easily proc Mark of the Assassin with Crescent Slash this Season than in Season 1, meaning she gains the healing received from the proc much more quickly, which is vital for a squishy melee. Spellvamp is useful if you are gaining back the heal from damage dealt very quickly. If you Q - > AA, usually this is very slow, since it relies on Akali's attack speed, which is quite slow to proc the MoTA debuff. Q - > E is obviously much better, since it is instanteous. It is also why just spamming Shadow Dance for damage is good in some situations, since Akali can use it to heal herself when she gets low.
If Akali needs toning down, it is most likely her sustain.
TDLR: The strength of spellvamp is currently stronger than in Season 1, therefore looking at Akali's sustain would be a good place to start.
