Eternal or everlasting life is a difficult term for the watchtower to use. It might imply that once you make it you would have everlasting life, which of course is only reserved for the leaders of the cult.
Time indefinite is much more appropriate as us peons might fail if we don’t do enough for them. Any good cult(or used car salesman) has to keep you interested enough to keep you in the sales funnel, while subtly telling you that you’re(or your trade in) is a worthless piece of shit.