It varies with every prospect. They'll have a very good idea if it's an elephant or not with the first well, but they won't have enough info to announce estimates with only one well.
We should know whether they are happy or not fairly quickly.
There are several tests that can be run after the initial well that give solid indications as to the size and extent of the field. Flow rate tests against a specific choke size and a BHP "Bottom Hole Pressure" build up test.
The tests described above will not give you everything but they could give youa a solid indication that you have an elephant on your hands. As Platina states you would have to drill several more wells and run the above tests on each well to determine with any kind of specificity.
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