Not very really...
In fact at 20000 feet you have the signal in huge area...
The hard work is to locate the exact position...and after that you need to descent at 4000 mt and retrive it...Not very easy...
I agree that in this case probably a longer life of the battery give a better chance to locate the FDR but 30 days remain a very long time and in the case of AF447 need to consider that the survey start after about 7 days, and furthermore we don't have a crash point...
I hope however that it can be found in the next few hours ...
Mak