Till your phone dies. Or your bank isn't compatible with mobile pay. Or mobile pay fails and you need to fall back on chip and pin. Or the transaction value is too high.
My phone battery lasts 2-3 days, if I was stranded somewhere I couldn't charge, with no wallet for 3 days, I would have bigger problems than whether I could make a contactless payment from my phone.
I already know my bank is compatible with mobile pay so this is a non-issue. If I was stupid enough to expect to pay with my phone without setting it up first then I'd deserve every bit of ridicule and inconvenience I got.
I can count on one wing how many times in the last 5 years (since I started using mobile payments) that mobile pay is not working but normal card payments are fine (hint - I don't have wings).
If I was planning on spending more than the transaction value, I'd take my wallet with me. Payment limit for mobile is £100 in most places, which is far in excess of what I would spend on a spur-of-the-moment impulse purchase.