How long to mature a starter?

Once it has peeked after feeding and is just about to drop again, that would be the time.

Rule of thumbs is 12 hours.

When starting from scratch, yes! Hamelman, recommends at lease 6 days, other a week or more depending on whether you add agents such as honey, juices or fruits of vegetable skins. Just because the yeast is active doesn`t means its mature enough to leaven bread. One indicator is a beautiful loaf, which is a hollow dome with a brick at the bottom. 

Just give it a go - pick a recipe and see what happens. You can test it by tasting it (mildly sour but not too acidic), smell it - smells faintly vinegary, but not like old socks and not like acetone [pear drops] either, and take a tablespoon of it and see if it floats (ie. full of gas bubbles)

Then just give it a go. The worst that can happen is that you waste a bit of flour, time and energy - if it fails, keep feeding it for another few days and try again.

Good luck with your new pet!

-Gordon

Give it a go. The worst that happens is you end up with a brick of baked dough. You could try kick starting your levain by allowing it to ferment in your oven with the pilot or viewing light on. I always do my preferment at a balmy 93F. It gets everything going really quickly, and I always have lovely oven spring. Mind you, I bulk for 4 hours at room temp, then retard for 12-24 hours before baking, which really helps with flavor development.

Something I've noticed, is that if you build your starter bulk up in a couple of steps, it really helps make active, happy yeasts. Try mixing 5g starter, with 10g flour, 10 g water. Wait 4 hours. Add 20g flour/20 g Water. Wait 4 hours, add 40g/40g.  If your starter doesn't double after the second feeding, take half out, and give it another 20g/20g. This method almost guarentee's a very vigorous starter. When I do this, my doughs always double in bulk during my 12-24 hour retard overnight.  

Nothing wrong with cheating on your loaf a bit, and adding a pinch of commercial yeast as a fail safe, either. I know i've done this before when I was first getting my starter up and running.  

You might just be trying to rush it too much. Sourdough is slow food. If your starter isn't doubling in 4-8 hours, it's not ready to bake with. Keep feeding it and letting it mature, it'll be worth it!