Powering up Haunter in Pokémon go with a legacy move

PvE: Like all other non-fully-evolved Pokémon, Haunter has no place here.

PvP: Haunter's max CP is 1963 (even lower with less than perfect IVs), which effectively limits its viability to the Great League.

Since its only realistic niche is in Great League PvP, you should power it up as close to 1500 as possible without going over.

However, you should also consider whether you may want to trade it in the future. Legacy moves are often very valuable, and since trading rerolls the IVs that could potentially put it over the 1500 threshold if you power it up too much, especially for such an old Pokémon that has a high likelihood of triggering a lucky trade. If you don't have any actual plans to use it, consider leaving it as is instead.

It depends on what you want to do with your Haunter.

Whatever you do, it will never be a top pokémon for gym attack or raids; for those you want fully-evolved pokémon for max power. You could still power it up to max but it won’t be helpful against most opponents.

You could use it in gym defence as a show-off pokémon, but people won’t notice it knows shadow ball unless they fight it—and if they do, their Tyranitar will probably crunch it before it has a chance to set off its first shadow ball. Still, if this is your intention, you would probably want to power it up to full.

As its max CP go over 1500 but not close to 2500, you can use it in Great League PvP or in Great League fights against team leaders. That requires powering up as close as possible to 1500 CP.

You can keep it for trading in case somebody else wants the legacy move. You might be able to trade it for a pokémon that has ‘more value’ for you. In that case, you might not want to power up at all as trading rerolls the IVs which might push it above the 1500 Great League viability CP limit.

Finally, you can just keep it in your pokémon list as a trophy to remind yourself of your achievement. In this case it is fully up to you how high to power it up although I personally would want top level just because.

Tl;dr: to whatever level you want it to go.