The right time is now. :D Do it already! Buy an airbrush!
In all seriousness, the time is when you can add a brush, compressor, spares, cleaning products and like £50 worth of paints to your basket and click the "Pay Now" button with some confidence. Do you really
enjoy painting enough that you think this is going to be worth the £300+ cost? Are you going to be able to learn new techniques and the muscle memory and hand-eye co-ordination required (it's a different ball game with an airbrush).
I am also slightly contratian to Cypher's view of the compressor.
Whilst a £200+ compressor is going to be fantastic and make things far more enjoyable, that sort of cost is unrealistic when you're just starting out. Maybe this christmas you get a nice airbrush and an okay compressor and next christmas you treat yourself to a great compressor. My £60 compressor (with a tank, may I add) is loud, vibrates in the non-fun way and overheats... But it's not like a turn it on and have to hide behind a blast door for fear of it exploding whilst I send in some specialists to douse it with liquid nitrogen to counter-act the heat coming from it like some episode of Stargate SG-1; it gets hot after about half an hour of use, sure, but it won't get hot enough to nope out for at least an hour and a half of use, more than enough for me most of the time and I plan accordingly.
SOMETHING TO KEEP IN MIND (which may have been mentioned already in this thread):
An airbrush is not a REPLACEMENT for a regular brush. It's just another tool in your arsenal. It's a major tool, sure, but at the end of the day the king of the hill is the good ol' fashioned paint brush. That being said, you can do huge amounts of the work using just the airbrush. I don't do many extreme highlights anymore favouring multi-stage lighting effects with the airbrush (This might be what zenithal lighting is? I don't know, that word I've seen mostly used to reference flat panels) which include a top highlight without making a line on the top most edge (I think it's often overkill and can spoil the look*, especially with my brush control). I feel I've gone off on a bit of a tangent now, so allow me to shut up.
* - an example of this, which is HIGHLY OPINIONATED AND NOT GOSPEL TRUTH, is Awaken Realms' Tau XV 109
. Before I say anything else, I do believe this is an absolutely stunning piece of work and well worth the money someone paid for it (which I won't even begin to think about; big numbers following a currency symbol scare me). However, the extreme highlights here are not to my personal liking and I would have preferred the piece without them, or at least with them toned done quite a bit; most of the highlighting effects are provided without these and are more than sufficient to my eyes.