As with any planning enforcement decision, the enforcement time scales are likely to be an estimate that will vary depending on the complexity of the case.
Some simple non-compliances may take a matter of open days to resolve without the need for formal enforcement action. Some more complex cases can take up to six months before they reach this stage. You have a Right to Appeal for any of these Notices.

For Formal Actions-

The amount of time it takes for formal action to be completed will depend on the type of enforcement action required and what stage in the process each case has reached. The later stages are likely to take longer than the earlier stages, with one exception being when a case is escalated to an Enforcement Hearing.
Compliant cases generally take three to six months, with the greater part of this time being taken up by any necessary pre-enforcement consultation. For non-compliance cases involving no major competing interests, the formal enforcement action should be completed within 6 months of receipt of the breach report, or within 9 months for complex cases that have required pre-enforcement consultation.
For more complex cases where a breach report has been followed by pre-enforcement consultation, the formal action should be completed within 12 months of receipt of the breach report.

In Major Competing Interests-

In cases involving major competing interests where an Enforcement Notice is necessary, each case will involve consideration of all the circumstances as well as the various rights that may have been established under previous decisions to grant planning permission. The formal action should be completed within 12 months of receipt of the breach report.
In cases where a formal decision to take Enforcement Action is required, the case will need to be considered by the Full Authority and it may also require pre-enforcement consultation. This process can take up to six months or longer in some cases and this time must be factored into the overall time frame.

At each stage of any formal enforcement action, the Local Planning Authority will consider whether all actions in respect of that case have been completed in accordance with their statutory duties before issuing a Decision Notice. It may be necessary to allow additional time at the end of this process for consideration and consultation by the Full Authority or committee before it is completed.

If an enforcement decision requires pre-enforcement consultation, or if the application is complex and required pre-enforcement consultation, the statutory time frame will be extended by a further three months to allow for this process. If any of these conditions apply to your case, you will be advised of the revised time frame in writing when you receive the Breach Report.

Provided that no further significant issues arise as a result of pre-enforcement consultation, it should not take more than three months to complete this process.

If you do intend to make an appeal or ask for a review after receiving the Decision Notice, they will extend the time frame by any additional time necessary to allow for the formal decision-making process to take place.