Someone is infringing your patent. Now you want to rush out and sue them! Let's stop and talk about that for a minute.
The best reason to wait: that infringer is promoting your product, which takes time, money and effort before it pays off. So, let them do it for a while, and then swoop in! You shouldn't wait too long, perhaps a year or two or three, although some case law suggests that you can wait six years to sue. Don't rely on that, of course - in a specific case your patent attorney should verify and recommend specific deadlines.
Let's just mention that there can be a number of defenses to a patent lawsuit. Such defenses depend on the answers to the following (and possibly other) questions, such as: Was the infringer actually the first to use it? Can your patent be challenged? Were you actually damaged? Is the alleged infringer's device/product really close enough to your patent to infringe its claims?
Now let's say there are no defenses to the lawsuit, and victory is absolutely certain. This is very hypothetical, of course, since there is always some uncertainly. But, assuming this perfect situation, should you still go ahead and sue? The answer may be “no” for a number of reasons!
So don't sue? Why not? Well, for starters, if the infringement was for a small number of products, you are unlikely to get meaningfully large damages. It is very easy to spend more money that you can recover. That's right, you can win the lawsuit and yet get little or no cash.
If the court finds in your favor, you can ask for actual damages, and/or lost profits, and/or attorneys fee, and/or costs, among other things. Notice I said you can ask! However, asking is not the same as getting. Worse, it can be difficult to prove lost profits or actual damages.
Even in a good case, with ample proof, it can be difficult to get a damage award that really compensates for the time and effort of litigation. Money is not the only cost of litigation. And, a damage award is not the same as actually receiving money, since defendants may not have assets to cover the damage award, may declare bankruptcy, and may even simply hide their money.
So, when should you sue for patent infringement? Simple – when it affects your business, or when the infringer is just making too much money from it and is worth suing. Yes, you can lose money suing an infringer that is poor, but if they are taking away your business then it may well be worth it.
Of course, the above is not legal advice. For that, you'd need to consult a patent attorney and then discuss the specifics of your case.