There are several ways to keep your trademark in China. The first is free and 100% bullet-proof: you must be sure that no one knows your trademark in China. You should only accept clients from China, advertise, sell, talk to people connected with China, and so on.
If you run an offline business, it's easy. You won't care about your trademark in China and it's unlikely that you will lose it there, and even if you do, well... You will never get to know this.
If this sounds difficult, since you have clients/suppliers/visitors from China, register your trademark. It costs US$299 for 10 years in Bonamark. You can't imagine how important it is.
We had a case in which our client had a trademark. Let's imagine that the trademark was "TRADE MARK". He had a partner in China who manufactured some products under our clients' mark. They worked together for several years and then suddenly, our client learned that his supplier registered his trademark! So the supplier registered "TRADE", "MARK", "TRADE MARK", "TRADEMARK", "MARK TRADE" and finally "MARKTRADE".
It took a year and US$50.000 to cancel some of the trademarks and the process is still ongoing.
So if you sell your products in China or plan to sell within next three years - Apply For Your Mark.
If you were attentive, you would have noticed that I mentioned 3 ways. Thus, what is the last way?
The last way will be to order a Trademark Watch. It costs slightly less (around US$180/year) and we will watch for any similar trademark in China. If it appears, you will be able to oppose it and prevent it from registering.
The conclusion is: if you work with China and want to have your trademark safe, register it.