Generally speaking, the more speed the better. With your arena team, you usually want to try and break the 200 mark. But you might want to slow a character down a bit to make sure it initially goes after another character. And some toons don't need a lot of speed. With GMY, for example, you can sacrifice speed for more offense. His abilities give him turn meter and bonus turns, and he is constantly called to assist on a typical Bastila or Revan team.
For arena squads, if you want to crack the top tiers you're probably looking at +120 speed on all characters from mods at a minimum. For most PVE stuff - LS/DS tables, assault battles, etc - +40-50 speed on your mod set is fine. Speed primary arrows add 30 (or 32 if you slice them to 6e), which makes that a pretty easy mark to hit. Raid teams fall somewhere in the middle, usually - +60-80 is probably enough to solo the Rancor or do solid damage on HAAT.
There are instances where you don't want tons of speed. Sith Trooper and General Kenobi, for example, taunt passively and lose their taunt after their turn, so they are often modded for low speed and maximum damage absorption. Some teams also take advantage of a slow Enfys Nest to serve as a makeshift tank, as she basically becomes invincible after taking a hit or two until the start of her next turn. Lastly, some Heroic Sith teams depend on characters' turns happening in a specific order, so you might need to slow somebody down from their normal speed to get the order right.
By and large, though, the correct amount of speed is "more." More speed = more turns = more win.
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There are instances where you don't want tons of speed. Sith Trooper and General Kenobi, for example, taunt passively and lose their taunt after their turn, so they are often modded for low speed and maximum damage absorption. Some teams also take advantage of a slow Enfys Nest to serve as a makeshift tank, as she basically becomes invincible after taking a hit or two until the start of her next turn. Lastly, some Heroic Sith teams depend on characters' turns happening in a specific order, so you might need to slow somebody down from their normal speed to get the order right.
By and large, though, the correct amount of speed is "more." More speed = more turns = more win.