At what point would you consider the best time to use Adwords?

[TYTD]

Obviously this is a subjective question varying from channel to channel but I imagine there are a few consistencies so I thought it best to ask here out of genuine curiosity. So; in the "How to be a recommended channel" thread the subject of Adwords came up and it got me thinking...at what point in a channels life cycle would you recommend looking into Adwords as a source of additional traffic. As Im already planning what steps to increase my traffic next year I was considering looking into getting into advertising like this once I'd had 40-60+ reviews on my channel (I upload weekly plus the odd extra when I can fit one in) so i'd have been doing reviews for about a year to a year and a half at that point. would that be considered too low ball for this kind of advertising or about the minimum amount? equally; has Ad words given any kind of boost to your channel? or has it backfired in terms of crashing your retention times?

Any advice would be greatly appreciated