[7777fps]

And by "don't take that into account" you mean "transact anything off chain" including other currencies, goods, services, etc.

[pat2man]

You can transact off chain. You just have to calculate the likelihood that your transaction will go through, and adjust accordingly. The cost of a 51% attack for 24 hours is a lot higher than the cost for one hour, so if you want for 24 hours the likelihood goes way up.