The short answer
If your site is the app's content, the technical build is usually days, not months: package the site, add the native shell, wire push or payments if you use them, and produce signed builds. What stretches a project to weeks is everything around the build, not the build itself.
Custom development from scratch is a different question entirely, six to eighteen months is the honest range for a fully custom app with real features, and if that is what you think you need, this article is not the one you want.
Where the waiting actually happens
Store accounts and fees come first. The Google Play $25 registration is quick; the Apple Developer $99/year membership can take days or longer because Apple verifies the entity. You cannot sign an ipa without it, so start there.
Then the reviews: a first App Store submission typically takes one to three days if nothing is rejected, Google Play is usually faster. Rejections add days each, and the common ones are avoidable, a real privacy policy, honest permission prompts, and a category that matches what the app does.
What you can finish this week
The parts that are yours alone: the app name and icon, the store listing text and screenshots, the privacy policy answers, and the contact details stores require. None of it needs code and all of it is on the critical path.
A good builder hands you signed files and a checklist for exactly these steps. If a provider quotes you weeks for what is fundamentally packaging, ask what specifically is taking the time.