Trademarks are fickle things. You only have them so long as people know and associate you (and only you) with that trademark. Whether or not you have a registration, your rights exist only exist if that trademark continues to distinguish you from everyone else.
So, if you have found a potential infringer do you have to sue or can you let it slide? The tough answer is that you have to stop them, and here’s why:
If you and your competitor are selling the same or similar goods or services, how will consumers know which is which? Even if your product isn’t exactly the same, consumers might think that you could have expanded your offerings into that area if its not too far off. If someone out there now sells products related to your own with the same name, you have to try to stop them if you want to keep your trademark distinctive.
If the other trademark is a little different, but still sounds the same, means the same, or looks the same as your own trademark and its used on the same product you have to stop them as well. Consider differently spelled trademarks that sound the same. If a customer is telling a friend about your great product, will they be able to hear the difference between “BISCOH Brand Product” and “BIZCO Brand Product,” or “BOSCO Brand Product?” Probably not. The longer those sound a like products are on the market, the less consumers can be expected to consider one the original and the other the knockoff.
Courts have also developed a doctrine called “laches.” That is basically a doctrine considering the fairness of bringing a lawsuit years after you knew or should have known about a legal problem. If you allow your competitor to continue building their market, investing time and money in their own infringing trademark and could have stopped them before they invested all that money, courts aren’t as willing to enforce those trademarks. There is no absolute cutoff point for laches to take effect, but as years go by, the chances you will be able to win your trademark lawsuit get slimmer and slimmer.
A registered trademark gives you a tool to keep competitors at bay, but it wont do it for you. Policing your trademark and occasionally bringing a lawsuit is the only way you can actually keep your trademark. No one is going to stop a competitor from infringing your marks if you don’t. The longer you wait, the less likely it is that you will be able to stop them.