Yours it a common problem because the pull up can really only be trained for by doing……pull ups. Using a lat machine can help a bit but it isolates the shoulders and I have not seen great luck in using them. The best method I have found for going from zero to one is the isometric hold method.
Get your chin above the bar anyway you can. Then fully weighted hold that top position for a count of at least 2 seconds then drop to the floor. Rest 1 minute and repeat til you can’t hold at all. Once you can hold for a count of 5 then use a lower iso-hold position: eyes even with bar and repeat the progressively longer count till you can hold for 5. Once there the use a lower position with elbows at 90 degrees and repeat same protocol.
No telkling how long this will take. Sorry. But I have seen it work well for those who can’t do one. Factors that play a role in the pull ups:
1) Body weight. This is the biggest factor.
2) Humerus bone length. Affects leverage. Short arms are better
3) Age. Adaptations are slower for older.