injector sleeves

Hi I bought this 2001 to tow our recently purchased travel trailer. Since it has 216K miles on it and was hard starting in the cold, I decided to put in new glow plugs and have the injectors rebuilt. When I went to reinstall the injectors I noted Anitfreeze in one of the sleeves, so I got a...

How often do Diesel engines need their injector sleeves and/or caps replaced? If its leaking fuel and you spot the issue, how long does it take to replace? Cost for this would be appreciated as well if you all know an estimate on how much this would run. Thanks!