My rule of thumb is 1,000.
It takes a thousand items to make a conversion really worthwhile. Less than that, you might as well hire a temp and have them key the data. That's 1000 of anything: Customers, Vendors, Items, lines in a bill of materials, ship-to addresses, open orders, etc. If you've got more than 1000, convert it.
If not, key it.
Why?
Well, briefly put, if you're moving from one accounting product to another, you can guarantee that the files don't look the same. They have different options. The item description is 40 characters in one and 30 in another. The item number can be 10 characters in the old and 22 in the new, or vice versa. And under the hood, in the database, they are called different things.
This means that conversion from one product to another--assuming that there's not an automatic conversion already built in--means writing a program, or at least building a map that says "put this data...over here." So there's always programming involved.
And 1000 items seems to be the break between when it's cheaper and better to code it vs. key it.
Just a rule of thumb, though.
