The honest answer is a range, and what places you in it isn’t mostly size — it’s custom code and content-model complexity. A big site with clean, boring structure moves faster than a small site with eleven years of clever.

The realistic bands

A modest Drupal 7 site — standard content types, common contrib modules, a theme worth replacing anyway: typically weeks. The migration itself is often the quick part; rebuilding the theme is where the calendar goes.

A complex estate — custom modules, deep entity-reference webs, workflows encoded in structure: months, run in increments alongside the live site. That’s not slowness; that’s the rehearse-and-reconcile discipline that makes the final cutover minutes instead of a lost weekend.

From 8, 9, or 10 to 11: usually far shorter — module compatibility and deprecated-API cleanup rather than re-platforming. Think weeks at the outside, sometimes days.

What actually predicts your timeline

Three questions beat any calculator: How much custom code is there, and does anyone alive understand it? Does contrib you depend on have a modern equivalent? And is the content model documented anywhere but the founder’s memory? The assessment answers all three in writing with a dated plan — which is why it comes first, and why its fee credits toward the work. Guessing is free and costs the most.