Hi Tim,
I'm afraid that it is impossible to estimate how long it might take to get the decision on your claim, it varies hugely from one area to another. Several months is not at all unusual.
How long until they make a decision? All people on IB will be transferred (assessments allowing) to contributory ESA, income and savings do not affect this (except for occupational pension in some situations). This is limited to 365 days for people put into the 'Work Related Activity Group', after this time the claimant will be means-tested to see if they are entitled to income-related ESA. There is no such limit to time in the Support Group.