An iOS 5 update to your iPhone or iPad can take an hour — or three, or four. I recommend you choose to “download only” first and then “update” later. In so doing you can untether your iPhone/iPad and leave iTunes to take its time downloading the file. If you choose “update” you cannot untether the device from your computer and you may be in for a very long wait.
There’s no way of knowing in advance how long the download will take. Therein lies the rub. Then there is the device update, which in itself may take upwards of another hour backing up, updating, then restoring everything.
I started my download at the office, shortly after iOS 5 was released to the public, when a crush of requests slowed servers to a three hour file transfer. It still needed two more download hours by the time I shut my laptop and took it home — where I resumed downloading (click to resume in the “download” folder). It took just another nine minutes to complete the download later that night from home. Then I synced and upgraded my iPhone, successfully.
If you’re doing this overnight and do not need to use your iPhone/iPad then you probably should go with the “update” and leave it to download and update.
To summarise …
1. Connect your iPhone/iPad to iTunes.
2. When the update is detected and offered to you, choose “download only” — not “update”.
3. Wait to determine how long iTunes estimates downloading the update will take. Decide if you’ll waiting on it, or come back and complete the update later.
4. Choose between …
4a. Walking away with your device while the update downloads: Eject your iPhone/iPad and take it with you. Return after the download has completed and sync your phone — choosing to “update” now that the file has downloaded in iTunes.
4b. Wait to complete the download and then update: If the download is a tolerably short wait, leave the device connected and then “Sync” after the download has completed — choosing to “update” now.
5. Patiently wait out the hour or so it’ll take to do the device update. Do not disturb the connection during this time.
My experience upgrading my iPhone 4 to iOS 5 went flawlessly. It’s a phenomenal upgrade. Many apps run faster and I have more functionality, what’s not to like?
Suggested link iOS 4 Upgrade Hangs on Backing Up iPhone