Having the right account and knowing your backups’ components is a critical first step in the backup process, but you also need to know the scope of the Microsoft Office 365 environment you want to back up. Which services do you plan on including in this backup scope? Knowing the components you plan on backing up will help greatly when it comes to capacity and resource planning.
You can expect a fairly static data growth when it comes to email because there are more limitations on data that can be exchanged in this form compared to SharePoint and OneDrive for Business where there are very little limitations on the type and size of data that can be uploaded. As you can expect, SharePoint Online and OneDrive for Business can have unpredictable data growth driven by everyday users. You can also see major spikes in these data areas when your end users start adopting these additional services to share collaborative data with both internal and external parties.