I've already had the discussion with her even though we only met in September!I definitely didn't want kids when I was 25, but still at that age I sort of knew I wanted them at some point. I always assumed from an early age that I would have them and that never really changed. I tried to put the Mrs off a little longer when we were both 30, but she wasn't really having any of it. We'd already been together for 6 years, were married and had a house, so she had a valid point. We also want to fit 2 kids in, so body clock was a factor too.
I would say that there is a good possibility you'll change your mind, but it will probably never feel like the right time. Don't think it ever really does for men. I'll admit that my immediate thoughts after 'arriving' in the Mrs bareback with no chemical backup for the first time in years was "Oh God, was that a good idea?!".
If you really think you won't want kids then better tell her sooner rather than later as it wouldn't be fair on her. In her mind she's thinking if you're not the guy, then she'll need potentially 6 months to a year to get over you, another few months dating, perhaps a year or two in a relationship with a wedding and a house before considering a baby, but of course you probably already know that.
We nearly ended it straight away when I told her my thoughts on the subject, but decided to carry on as I'm a bit younger and so she knows I'm likely to need more time to get into the frame of mind of wanting kids. We haven't talked about it since but I suspect it's going to come up again next year if everything is still going well!

Also, I hate the Tweenies.