When would you choose to start corrupting pot over typical starting items?

Generally speaking, your going to get more benefit for corrupting pot, when you can afford to back more often. And that usually means your running TP.

If you can back more often, you simply get more of those refills. This can be particularly useful on some AP champ who combined it with the Dark seal, because it boosts the effectiveness of the potion.

When you think the only way to sustain in lane long enouigh to keep xp even with the laner is to have pots on hand

when you plan on trading a lot or going for an early kill as a champ that uses mana

If you expected to get bullied, are planning to bully, or have good synergy with it (Dark Seal + Corrupting Singed for example).

I pretty much get it on any champ that can/will start with refillable, or any lane that i know is gonna be trade-heavy (lots of backs, lots of bought potions). Even on supports if i had to back early because of trades (rare though, i prefer to rush a supp item, sight stone, or something like redemption/ardent)

The way I look at it is: Both you and your opponent will be buying items all game. But, which one of you will have a free heal all game? Potions win trades (sometimes)

Buy it when matched with/using the following:

• poke heavy match ups
• mana heavy top lanes
• trade heavy match ups
• just plain up need sustain (generally goes with off-meta top lanes)