This is a tough one to answer because there so many variables involved. You need to think of the speed of the machine that is the source, slow disk drives will be a definite hindrance to a quick migration. There is the number of mailboxes to be moved. The size of each mailbox is a factor. The actual speed of the network comes into play as well. If you are doing the migration during the business day, the speed you get can be quite variable, while at night, it can be more predictable.
The best you can probably give them is a rough guess which would be based on the move of a number of test mailboxes of various sizes, number of folders and number of attachments to reflect a real life mailbox in that organization. Time how long it takes to move those mailboxes, then you can work out a rough guess. Stress to your customer that is all it is, a guess, citing at least some of the considerations involved. If you come in under your guess, you will look like a genius. If you come in at over, you will not be fit to eat supper with the dog, but this is probably the best you can do for an estimate.