I think it's pretty clear they would have been engaged until Captain Wentworth earned enough money for them to marry and not risk Anne being left a young poor widow with children to support. In Chapter 4, Lady Russell's thinks the following about the engagement:

Anne Elliot, so young; known to so few, to be snatched off by a stranger without alliance or fortune; or rather sunk by him into a state of most wearing, anxious, youth-killing dependance!
The "or rather" indicates to me that LR is here correcting herself, Anne would not be snatched off into marriage right away but have to spend an indefinite period of time waiting anxiously at Kellynch for Wentworth to earn enough money.

And later in the same chapter, Anne thinks:
She was persuaded, that under every disadvantage of disapprobation at home, and every anxiety attending his profession, all their probable fears, delays and disappointments, she should yet have been a happier woman in maintaining the engagement than she had been in the sacrifice of it

If they were planning to marry right away, rather than have a long engagement, I would expect the phrasing to be something like "a happier woman in being married ..." rather than "maintaining the engagement."