The question is like asking "how long is a piece of string?" but I'll try to give you something concrete to hang your hat on.
A "typical" migration from, say, a legacy monitoring tool to Datadog, or from one ticketing system to Jira, can take anywhere from **3 months to 18+ months**. The variance isn't about the data volume; it's about the *process and people*.
Here's what actually eats the calendar:
* **Discovery & Cleanup (25% of time):** You find out your data is a mess. Duplicate users, orphaned records, inconsistent custom fields. You can't migrate garbage, so you spend weeks cleaning it.
* **The "Oh, we also use it for..." (40% of time):** The team using the tool has built five undocumented workflows and three critical reports you knew nothing about. You now have to rebuild those or change processes.
* **Testing & Dry Runs (20% of time):** You run a pilot with a small team. They find 47 edge cases. You fix. Repeat.
* **Actual Cutover Weekend (1% of time):** The final data sync and switch is often the shortest, simplest part.
Real example: We migrated from ServiceNow to Jira Service Management. The sales pitch was "3 months." Reality?
* Data cleanup & mapping: 6 weeks
* Rebuilding approval workflows they "forgot" to mention: 4 weeks
* Testing with pilot group: 3 weeks
* Staging dry-runs and rollback drills: 2 weeks
* Final cutover: 36 hours of a weekend
Total: **~4 months**, and that was considered *fast*.
The single biggest predictor of timeline isn't the tool. It's whether you have a dedicated, full-time project owner with the authority to make decisions. If this is someone's "side project," double whatever estimate you get and add budget for aspirin.
Cloud costs are not destiny.