I'd say it is zero days minus 1.

You have backtested a system, and it shows a profit.

Ask yourself, a) "Is it likely that I'm the only one to have discovered a) this successful system" and, b) Can I be sure that this mechanical system can continue it's success given that it is dependant on the behaviour of a 600kg horse, takes the trainers and owners motivation into account ........ etc.

Given that we (and how many "we's" are there working on this?!) have the ability to instantly analyse all sorts of information and backtest all the variables ...... then what are the odds that you have found something that works, that no-one else has found in 50 odd years.

Hypothetically speaking, say you have found a mechanical system that has shown a profit when backtested.

How long will it last?

and more importantly!!!!!!!!!

Your have discovered this successful system, are committed to it, continue to fine tune it ........ It slowly but surely doesn't produce the ODDS required for you to show a profit ......... When do you bail out?

I'm certain you would follow it into a loss.

So you've followed a successful system into a loss, where to now?