Of course there is such a thing. It's one of the key economic indicators used to gauge the overall state of the economy. It all comes down to your personal financial liabilities and commitments and what's left once all the bills have been paid. I personally have no wife and kids, no loans or outstanding finance on big ticket items like cars, etc., so my disposable income is around 60% of what I earn overall. However I do live in London, so according to those who dabble with statistics my disposable is likely to be higher then elsewhere in the country.
I'm usually careful with my expenditure and save around 40% of what I earn each month, but that's only because when I was younger and getting paid fuck all, I lived life in the red from one month to the next, literally saving maybe £20 a month to slowly pay-off a £2,000 overdraft. And as you say, it only takes something out of the blue like a massive dental bill for root canal treatment, or something, to set you back again after month's of saving. My situation was only improved when I was made redundant, received a large payout and was lucky enough to find another job within a few days. Since then I found that managing money, expenses and creating budgets can actually be quite good fun.