Usually it would take a day to survey your existing panel, check which materials are available locally, then price the job and time to fix it .
When we have a contract and payment the “first day” is what I call a “preps day”. This includes getting materials, getting permit, labeling panel, in general getting ready for the panel change.
I have been an electrician since 1988 and have never failed to get the power back on the same day in an occupied residence when doing a main electrical panel upgrade.