For a basic first version, they should be able to knock it out in a weekend. If they can't, it's because your first version is too complicated.
Happy to discuss more about working with developers on a call! I'm a CTO with experience managing developers.
The best approach would be to split this project into smaller tasks and ask dev team to prepare a ballpark estimate. Can't provide accurate answer as it really depends on their individual skills/performance. However my iOS team would be able to do this roughly within 1-1.5 months. Please let me know, if you need any further support. Thanks.
The answer depends entirely on the details of your application. The more features and pages you have, the more time it will take. The higher the quality you're looking for, the more time it will take. It really takes an experienced developer going over the to determine how long it will take. If you're still looking for help, I'd be happy to go over the details if you want to schedule a call.