Well...my car engine blew up on the M25 the other day....first there started this big rattling noise, like very loud pinking, then a bad hot oil smell filled the car, then smoke began to **** out the back. However the car carried on for maybe 10 miles, loosing power, but still going, knocking getting worse and worse. Eventually I pulled off the M25 (luckily, just before the Dartford tunnel

else we would have closed it), and stopped, the engine would no longer tick over, and flames appeared under the bonnet (oil fire). Probably lasted 10 minutes in all, possibly longer had it needed to.
So it depends how it goes I suppose. My problem was due to lack of oil, something ruptured and blew all the oil and water out (yea, I ignored the 'low water level' light as 'it always does that')...Luckily my motor was worth all of £50 in the first place, so I gave it to the recovery guy in lieu of payment.
Cheers
EA