When to upgrade?

I've been fairly serious about photography for about a year now. I'm learning as much as possible from many sources and definitely have room for improvement. I shoot a variety of subjects...a little of everything and anything.

When do you guys decide it's time for a lens upgrade? I've come across a reasonably priced 24-70L used. Initially I thought about waiting on any upgrades because I don't really NEED a new lens in a range I already have covered with the sigma 17-70. I'm in no hurry (except I might miss out on this used lens, but I'm sure more opportunities will come up), but I would love to have this lens. Or should I wait an indeterminate amount of time until I really feel my equipment is hindering my progress? And who knows, by then, maybe a new L will exist that is even better. (I also will eventually upgrade camera body to full frame). Any thoughts would be appreciated!

When your current lenses aren't doing what you want them to.. when you want something they can't offer - time to upgrade. If you already have a midrange zoom that you're happy with why not expand your photography in another direction? Macro lens, flash unit etc..

Before I say anything else, let me say that I am a gear junky - thanks in no small part to my wife and this forum...

That being said, a great lens like the 24-70L can never be a bad investment. If you have the money to buy it and you are getting a used one at a good price, then it's resale value should be approximately what you paid for it. With that in mind, I say buy it, try it and see if you like it. There are certain things you can do with a constant f2.8 lens that you can't with the Sigma 17-70. Not to mention the luxuries of "L" like the weather proofing, build quality and speedy focus.

I see you have the Tokina 12-24 so if you decide to buy and keep the 24-70 you have a full range from 12-200, and you could sell your Sigma 17-70 to help fund maybe a flash....

As far as "when" to upgrade, well Jim G is right... but that doesn't mean it's not fun to get new gear even though you may not NEED it <grin>