A spokeswoman for the SIB said that it would be a rolling fund with no fixed closing date.
"We can’t say how large it will be at the moment because we’re still speaking to potential investors," she said. "It will not be government money, so we will have a lot more control over the criteria, and we won’t need to offer a fixed window to apply.
"It may well be that it remains open and is topped up over time."
She said the SIB had not finalised the size of the fund and was not setting a minimum or maximum amount for which applicants could apply.
To put in an expression of interest, an applicant must be able to take the money later in the year, must be able to pay between 5 per cent and 8 per cent interest and must be seeking a loan for between five and 10 years.
The spokeswoman said the fund would target "the kind of people who have borrowed money from us before", and there were no plans at present to open it to profit-making businesses.
The Social Investment Business said in 2012 that it would open one or two funds worth £10m, and would target the creation of a much larger fund if that was successful.