How long can I tube feed?

Hello, well my little babies are doing pretty good. They are now 11 days old and have started opening their eyes!! I have been trying to get them on the bottle or atleast to try and take kitten milk from a syringe or eye dropper...nothing works! So I'm still tube feeding them. I am concerned about the continuous use of the tube and their throats...I can't find information anywhere on how long it is considered safe to tube feed these kittens. Any suggestions? My vet seems to think it is ok to continue, but I know with other animals there is a 2 week maximum when tube feeding. Thanks!