A timeline estimate for a data migation can have a LOT of factors in it. Some things to think about:
What kind of data is being migrated from the source system? Are there different "types" of data or is it all one type? Is it metadata only or is there also content? Are there associations/links between the objects (documentation relationships, BOMs, etc)?
What about version history? Are you looking to migrate all versions of every object or just the latest version? Or, are there more complicated rules for capturing what to migrate?
Specifically how much data is there to migrate? Instead of "25 years worth", could you put together a quantity of actual records of data?
Once the data is understood, how would this data map to Windchill? What Windchill objects should be used and why?
Speaking of mapping, what initial configurations of Windchill need to be present in order to satisfy the source data (context configurations, OIRs, ACLs, workflows, lifecycles, soft types, attributes, etc)?
What migration method will be used? There are several ways that data can de migrated into Windchill depending on what the requirements are.
These are just a few questions of several. I'd be happy to have a conversation with you on it - if you are interested, feel free to send me a direct message and we can coordinate a call. I don't know where you're at in your process (looking at vendor quotes, maybe?) but my comany is very experienced in migrations into Windchill and if you were interested, we could scope this for you.