When starter is ready to make new starter?

Daniel DiMuzio has a nice photo showing ripeness of levains.

It also appears in his book, Bread Baking, An Artisan's Perspective.

then this would be when your baby can double (or more) in volume within about 8 hours (or less) consistently over several feeds. This would usually take a week or so while numerous other factors would also play into it (temperature, feed ratios, feed frequency, type of flour used, etc.).

But even then, you may want to keep feeding the new starter for a week or two more so it can strengthen and grow some "character". Although it may bubble proficiently at this early stage, it won't really develop it's flavour for a while yet.

If, however, you're talking about an already established starter and asking when it's ready to use, then that's when it's just hitting it's peak and before it starts to fall back down, having eaten up all the food in the jar. Again, this applies to 100% hydration starter. Stiff starter or very liquid starter are different animals altogether.

http://www.sourdoughhome.com/startermyway.html

I found this helpful when trying to visualize the stages a starter should go through.  Also, one can "move" between a stiff and liquid starter as they need to.  Some folks prefer to have one of each, some only one type.  I have a liquid starter (100% hydration) that I keep in a glass jar with the lid barely on in the fridge.  Check out the photos in the above post.  Hopefully that will help!