A full caster should have Quicken Spell as soon as they have 5th-level spell slots available.
If you have traits that allow you to cast a certain spell with Quicken Spell without increasing the actual level of the spell slot used in the process then you might need it earlier but you should otherwise be fine if you wait until level 11 or 13.
My rule of thumb is get it once you have 6th level slots, barring special circumstances.
Define necessary. It certainly isn't required for a wizard to be good, though it is useful depending on how your DM plays (ie. no warning of encounters)
Throughout 3.5 and PF, I've run approximately 5 mid-to-high level spellcaster characters that were all quite successful and on the high-end of practical optimization .... without the Quicken Spell feat.
However, most of them acquired Quicken Spell metamagic rods at some point ... but they were all still able to successfully and consistently maximize the use of their Swift Actions via appropriate magic items and casting good spells that intrinsically had a casting time of 1 Swift Action.
So you don't *need* the feat. But at approximately levels 9 and higher, your spellcaster better have a variety of useful options to CONSISTENTLY spend his Swift Actions for maximum benefit. And of course, you can always make or acquire Quicken Metamagic Rods (which have their own unique set of pros & cons).