There are two breath tests that are given by the officer. The PBT (portable breath test) given in the field and the breath test given at the station (on the "Datamaster" device). The PBT is voluntary and can only be used to help the officer gather probable cause to arrest. There is almost never a good reason to take this test. The breath test given at the station should be done within two hours of driving, but may still be admissible if given later. You should exercise your right to an attorney and get legal advice before deciding whether or not to take this test.
The evidentiary breath test, conducted on a BAC DataMaster, will be conducted after you have been arrested at the station. A lawful arrest is a prerequisite to a request for an evidentiary breath test under RCW 46.20.308. While there is the "two hour rule" which states that a test must be conducted within 2 hours of driving, a test taken after two hours can still be used as "other competent evidence of impairment" under RCW 46.61.506 and RCW 46.61.502. Typically, a test is conducted at a station approximately an hour after you have been pulled over. The test given on the side of the road (PBT) is not the evidentiary breath test, but can be used by the officer in making an arrest decision.