how long it takes to validate an App before it is uploaded in the Store?

When I worked on my company's mobile submit team I was responsible for uploading and releasing 3-5 apps every week.

After submitting an app for review, it typically takes 1 week to pass through Apple's review process (pending approval).

Though I have seen it take a quick as 3 days and as slow as 2+ weeks as it is slower around the holidays.

We had 2 week release cycles for apps in order to account for any possible rejections from the Apple App store.

After the app is approved you can set the app to release on the App Store, that usually propogates in a few hours.

Hope this helps,

DT

Apple changed to a target of one to two days, new apps and updates, back in May, 2016 and has this to say about that here: App Review Overview and FAQs

App Review Status

Once you've submitted your app for review, you can view its status in the My Apps section of iTunes Connect or on the iTunes Connect App for iPhone and iPad. Review times may vary by app. On average, 50% of apps are reviewed in 24 hours and over 90% are reviewed in 48 hours. If your submission is incomplete, review times may be further delayed or your app may be rejected. Once your app has been reviewed, its status will be updated and you will be notified. For details on viewing and changing your app’s status, read the iTunes Connect Developer Guide.

“On average, 90% of submissions are reviewed in less than 24 hours. You’ll be notified by email of status changes. You can also check the review status of your submission in the My Apps section of App Store Connect or on the App Store Connect app for iPhone and iPad. If your submission is incomplete, review times may be delayed or your submission may not pass”- apple website.