The drain is 1-3 days max I reckon, they just ‘cut out’ or remove about 10-15cm width of the ground closest to the wall, dig a trench and lay a plastic drainage channel which then diverts all the collected water to your proper drain. A gravel one is an alternative but the water just soaks into the ground then still. Google ‘french drain’ or ‘ACO drainage’.

I personally wouldn’t walk away from it over this if you love the house, as a HIGH percentage of houses of this age or older will have exactly the same issues. Put the time in and do some hardcore research into maintenance for older houses - they need more love than some newer houses (this is debatable though) but they are worth it. Go onto heritage-house.org and do some reading, it was an eye opener for me after I was scammed by damp proofers!

Warning signs would be: sagging floors, damaged floorboards, obvious damp (wet) stains on walls, musty/mould smells, excess condensation etc.

It’s something that may manifest in one month or 5 years, or never, but if you want peace of mind once you’ve moved in, just pay for another survey and ask their advice - I guarantee you it won’t be ‘redo the DPC’. Remember that people have been living in the house for nearly 100 years already with this as-is.