It depends almost entirely on how much your layout code assumes a fixed screen size.
An app already built with adaptive layout and an iPad target is often a few days of audit and refinement — the inner display is regular-width in both dimensions, which is the environment an iPad layout already targets.
An iPhone-only app with hard-coded dimensions, orientation locks and custom navigation containers is a larger piece of work, because the fixes touch layout code throughout rather than in one place.
Games and apps with custom-drawn full-bleed surfaces sit at the harder end, since resolution changes mid-session and the fold has to be handled explicitly.
Estimates by app type
Through the point at which an app looks correct rather than merely functional:
| Your app | Realistic effort |
|---|---|
| SwiftUI with an existing iPad layout | 1–3 days |
| UIKit on standard containers | 3–5 days |
| iPhone-only, fixed layouts | 1–2 weeks |
| Custom navigation or custom bars | 2–3 weeks |
| React Native / Flutter | 3–7 days |
| Games, custom-drawn surfaces | 2–4 weeks |
The biggest predictor is not framework — it is whether you already had an iPad target. Teams that did are usually surprised how little is left. Full breakdown in what to fix first.