You're right--timing is a tough one to call.
In people's minds, timing is everything and where one person may get your message immediately others may take a bit more time to recognize and pay attention to your commercial.
How do you measure your campaigns? Are you tracking any stats?
For production, see if there's a local film school that can help--they are usually a lot cheaper. I went with one here in Las Vegas recently and had a 60 second commercial quoted at $500. That's cheap. Local universities are always looking for projects like that too. There are film school students looking for the opportunity to do work so they can build their portfolio, why not give them a shot at helping your business and give them a letter of recommendation?
If there is a way to say an average time is there, it should be through the media outlet (TV/radio station) that you are working with, and by your own budget limitations you should have the control to decide what will work, and for how long.
If you're not getting any response in the first two or three weeks of advertising, perhaps it's time to scale back or hold off until you begin to see a trickle effect come in.
Once that's there, you can begin to reengage your audience and start to get them more familiar with you. If no trickle effect begins you know your message was either not clear enough, or just not targeted to the right demographic.
The big guys are already branded and know that when they come up with anything new they are going to get an expected response. Either "Oh cool, I'm there!" or "Sheesh, not again."
In either case they can adjust if they need to. If you have no track record of your marketing campaigns it's going to be an A/B test for the first round.
Check your budget to make sure you can handle a good solid 30-day run--it may take that long to get any real feel for the response to your marketing message. Then it goes back to have you done your research? If so, your average 30-day run should be sufficient to tell whether or not your marketing message is clear enough to get you sales.
Also, don't forget your response rate to your campaign has to make sense. If your investing looks more like spending it may be time to change media or at least vendors.
Remember the sales rep is trying to make a living too. Track everything! Don't leave anything to chance.
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Question added to topic Marketing • February 1, 2010
How long should I run a TV ad before changing the content?
Since producing TV commercials is costly, is there an average length of time I should plan for a TV commercial to remain effective before it reaches a point of diminishing return?