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I've got a lager (Wyeast 2206) that I've just taken out of primary fermentation (at ~8C, for 10 days) for a diacetyl rest (at ~20C). I picked the time to do it based on "How To Brew" suggesting I wait until bubbling slows to 1-4 bubbles / minute. It was about 18s between bubbles.
It's now at 20C & is bubbling away furiously, so I'm concerned I may have moved it too early. How can I tell if it is too early? Does it matter? What's the impact and can I recover from it?
NEVER use bubbling as a measure of anything important. You MUST take actual gravity readings to know where your fermentation is.
As a general rule, you should do a diacytl rest when you are about 70-80% done with fermentation. So for a "normal" strength lager (1.050-1.060 OG) I'll start the diacytl rest when the beer hits about the 1.020-1.022 range (assuming I'm expecting it to finish around 1.012-1.014). This is usually around the time when the bubbling starts to slow down a touch, and the krausen starts to droop a bit, but you can't really tell without taking gravity readings. Since lagers ferment a touch slower than ales, the window for the diacytl rest is a bit bigger, and I've never had a lager that dropped too fast so long as I was taking samples every few days.
Flavor wise, if you warmed the lager up too early, you will get some of those undesirable flavors that make Steam Beer (like Anchor Steam) taste like they do. Its hard to describe, but its like a "rough" flavor to me, and you could potentially get a touch of fruity esters as well. However, if your beer started fermentation at the proper temps, and you just warmed it up a touch too early, these flavors should be minimal.
One last thing to remember is basic physics: cold liquids hold more C02 in suspension than warm ones. If you had a beer at, say, 50F and you brought it up to 70F after fermentation was 100% done, you would see bubbling. But NOT because of new fermentation, but because of the fact that some of the C02 in suspension at 50F will bubble out at 70F and make your airlock jumpy, just like during fermentation. In fact, this might be exactly what you are experiencing. Your beer might be right at the proper gravity for the d-rest, but the warmer temps ALSO make a few extra bubbles of C02, so it looks like fermentation is going on more vigorously than it actually is.
tl;dr; - Your beer will most likely be great, but next time, take gravity readings as the fermentation starts slowing, and move to a D-rest temp (63F-70F) when your fermentation is 80% done.
