Here's a few questions for you.
- You say you are a .NET development agency. Do you have experience in the CMS or DXP space?
- Have you ever worked with Kentico 12 MVC or Kentico Xperience 13?
- How good is the client in communication so far? Do you feel confident in their ability to explain what their current Kentico 12 MVC application does and how they use it?
- How confident do you feel about being able to analyze a Kentico 12 MVC code base and identify what is standard and what is custom?
- Are they using commerce?
- How many ASP.NET MVC 5 -> ASP.NET Core 8.0 conversions have you done?
- Does their project rely on any 3rd party or pre-compiled libraries?
- Are they using any of the marketing features beyond forms? (like marketing automations, emails, or campaigns)
- Have you reviewed the Kentico Xperience 13 upgrade documentation and do you understand everything you've read there?
Many Kentico partner agencies could answer yes/good/confident to many of those questions even if they haven't done the Kentico 12 MVC -> Kentico Xperience 13 update for a client.
My gut feeling is definitely not 2 days, and anywhere from 2 weeks to 2 months depending on your background knowledge and quality of client relationship.