According to adsense blog Starting today, you will have immediate access to your AdSense account once you sign up. we all know that Previously, when you apply into the adsense program, it takes days as you need to wait for your account information to be approved before you could access your account and place ad units on your site.
With new improvements, you can create ad units and place them on your site right after signing up for AdSense.
To get started with AdSense:
The previous process requires you to takes 48 hours for google specialists to complete a review on your website after which you can add the code to your site.
NB: Until the review is complete and your site approved your ad units will remain blank.