It is a difficult question to answer and it really does depend on both the Business Angel's and the entrepreneur's objectives being aligned. What are you looking for? What are they looking for? When they match, you might just have a deal.
Entrepreneurs need to carefully consider the mix of time, relevant expertise and cash that they require. You might raise as little as £5k or as much as £150k from a single Angel. Larger sums, sometimes over £1m can come from a syndicate of Angels investing together and this is an increasing trend.
An interesting statistic. Recent research indicates that out of every 10 investments, 6 will simply fail to return the amount of the investment, 3 will do 'okay' and perhaps 1 will return around 10 x the investment. Angel investing is therefore high risk for the Angel, and not always as lucrative as you might expect.
Engaging an Angel with experience in your sector, and using that sector experience can make a big difference to those odds for both of you.
Another organisation, UK Business Angels, believe there could be as many as £18,000 Angel Investors (sometimes known as Informal Investors) in the UK.
So, if I was really forced to judge the length of this particular piece of string, I would say that investments are commonly around £50k and supporting effort can run to perhaps a day or two a month, sometimes decreasing with time.