Many advertisers considering remarketing campaigns are concerned about inundating their visitors. In fact, you could alienate your customers by spamming them with ads that constantly follow them around. To protect your brand, you can set frequency caps in AdWords, which by default Google does not enable in your campaigns. However, it's easy to set this up and it can make the difference between a highly effective remarketing ad and a customer never purchasing from you again.
Decide how often you'd like your ads to show to past visitors. Advertisers differ on how often they like to show their ads to past visitors, and I recommend testing different frequency caps to find what produces the best results. There is no right or wrong impression cap limit, and you'll want to test to figure out what's most effective for you. I recommend starting with 3 to 5 impressions per day and adjusting from there.