Four stages. First, discovery and an architecture review: which systems are in play, who owns which field, and what happens today when somebody changes something by hand. Second, solution design — the field mapping, error handling, authentication, and whether the flow is synchronous or not. Third, development and testing against a simulated partner. Fourth, deployment with monitoring and alerting.
The time it takes depends almost entirely on the other side. A well-documented API and a narrow scope is two to four weeks of work. Where the partner’s documentation is wrong or there is no sandbox, most of the time goes on establishing how the partner actually behaves rather than on code — and we say so upfront rather than in week three.
When choosing a partner, ask what happens when the other side is down, how the same order is prevented from being processed twice, and where you can see that the integration is working. If the answer to the last one is “the customer tells us”, there is no monitoring.