Great question. To help give some guidance, we've created a handy chart for you to reference the next time your writing some ad copy.

The 30 Second Spot Word Count Guideline

30 seconds, 65 words = lots of inflection, sincere read
30 seconds, 78 words = typical, straight read
30 seconds, 90 words = fast read
30 seconds, 100 words = hard sell, maximum speed read

The 60 Second Spot Word Count Guideline

60 seconds, 130 words = lots of inflection
60 seconds, 155 words = typical, straight read
60 seconds, 180 words = fast read
60 seconds, 200 words = hard sell, maximum speed read
Note that numbers (such as a phone number), symbols (@ symbol or www) , etc., count as one word each.
If you need help with your commercial scripts, you don't have to start from scratch. There are plenty of resources online or simply listen to a few radio commercials or tv ads to get the ideas flowing.
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Source: http://blogs.voices.com/castingvoices/2007/10/good_length_for_ad_copy.html