ANSWER
How long does it take to retire a legacy system?
Reviewed .
It depends on the size of what is being retired. A single task with comparable input and output typically takes a few weeks from sandbox to live; a workflow with integrations lands around a month; a full application takes months and requires a mandatory parallel-run period before shutdown. These ranges are typical estimates, not a promise: your number comes out of the Diagnostic, measured on your real volume, rules and data access.
Step by step
- Size the work: one task, one workflow with integrations, or the entire application.
- Measure what really drives the timeline: number of business rules, systems touched and quality of data access.
- Reserve the parallel-run period in the schedule — it is what proves equivalence, and it cannot be compressed without losing the proof.
- Agree on the cutover criterion before starting; without it, the parallel run becomes an eternal pilot.
- Treat the first retirement as a reusable pattern: the second is usually faster because the House Rules already exist.
How an agent does this
The AgenticosCore Diagnostic returns the likely size and timeline range in 3 minutes, with no sign-up. The public sizes (P, M, G and XG) are at /en/sizes/, each with scope, typical timeline and what requires human approval before cutover.
Frequently asked questions
Does the parallel run delay the project?
It is the project. Cutting over without it transfers the time you saved to the production incident — with interest.
Can more people speed it up?
A Legacy Exit timeline is bounded by proof, not headcount: the comparison period must cover the process's real cycles, like closing and peak load.
What if the candidate does not converge in time?
The equivalence criterion shows that early — and you learned it for the price of a proof, not the price of a migration program.
Primary sources
- Managing legacy technology — GOV.UK