Vendor timelines are sales collateral, not project plans. They assume perfect conditions, zero internal friction, and that their tool is the only variable. It's fantasy.
A full stack rebuild touches procurement, security, legal, data migration, and training. Each has its own queue and potential for rework. If you're replacing a core system like your data layer or CRM, triple the vendor's estimate as a starting point.
From a FinOps perspective, the real timeline is dictated by your contract cliffs and cost of parallel runs. You need to map out:
* **Contract End Dates:** When do your current licenses truly expire? Can you exit early without penalty? This is your hard deadline.
* **Parallel Run Period:** Budget for running old and new systems concurrently. For critical systems, this is 1-3 months of double-paying. This period often gets underestimated and blows the budget.
* **Internal Resource Contention:** Your team is likely maintaining the old stack while building the new one. How much bandwidth do they actually have? 20%? That alone stretches a 3-month project to 15.
Where things slip: security reviews, custom integration work the vendor "discovered" during scoping, and data migration validation. The last 10% of data cleanup always takes 30% of the time.
So, start with 9 months. Break it into phases: contract/legal (1 month), pilot/proof-of-concept (2 months), core migration with parallel run (4 months), decommissioning old stack (2 months). That's a realistic, padded timeline. If the vendor balks, ask for a detailed statement of work that includes every internal dependency and approval gate. They won't have one.
Your cloud bill is 30% too high