The Real Timeline for a MarTech Stack Migration vs What Vendors Promise

Every vendor migration timeline I've been handed in a sales deck has been some version of six to eight weeks. Every migration I've actually run, across CRM platforms, CDPs, and marketing automation tools, has landed somewhere between four and six months. The gap isn't dishonesty — it's that the vendor's timeline is measuring a different project than the one you're actually running.

What the vendor timeline measures

The six-to-eight-week estimate usually covers data import, basic configuration, and a go-live with clean sample data. It doesn't cover the part where you discover your existing system has eleven years of inconsistent field naming, three different definitions of "active customer" depending on which team you ask, and seventy automation workflows nobody fully documented that all need to be rebuilt, not just copied over.

The vendor is quoting you the time to move data. The real project is the time to decide what the data should have meant all along.

Where the real months go

Weeks 1-2:   data audit, surfaces inconsistencies nobody knew existed
Weeks 3-6:   field mapping and data model decisions, the real bottleneck
Weeks 7-10:  workflow rebuilds, not copies
Weeks 11-16: parallel running old + new system, catching discrepancies
Weeks 17-20: cutover, cleanup, and the inevitable "one more thing" list

The parallel-running phase is the one vendors almost never budget for, and it's the one I'd never skip again after watching a client cut it short once and spend the next two months reconciling data that diverged silently during a too-fast cutover. If a vendor's timeline doesn't include a parallel-run period, ask them directly where data validation happens — the honest answer is usually "during go-live," which is the worst possible time to find a mapping error.