Do *not* wait until you have started to exceed your estimate before you notify the client that the release is running late.
Keep them informed and avoid conflict by sending an email like this:
To: Mr Northwind
Figure: Good Example - A sample of an email that informs the client that the estimate will be exceeded
Cc: David (Project Manager)
Dear Bill Northwind,
As per our conversation, The Data Migration task in Sprint 3 will take longer than expected and so looks like it won't be delivered this sprint. The data migration of the Coverage Data is more complicated than originally anticipated because an external database (Media Disk) also reads from the coverage table. I am revising the estimate accordingly to 32 hours.
As soon as you realise that any of your estimates are likely to be exceeded by a material margin (about 20%), then let the customer know ASAP by phone and by email (using the 'as per our conversation' rule). This will ensure that the client is fully aware of any problems and has a chance to decide an alternative action.
Never keep the client in the dark when you exceed your estimates, it will only arouse suspicion and mistrust when they see the project deadline woosh past.
Note: For Scrum projects, you should keep an eye on your burndown chart during your daily standups to see if you are on track to finish all the work in a Sprint.