Our BoltHold asphalt anchors feature an internal thread. The thread extends just short of 1 inch inside the anchor, which is all you need. There is no advantage to using a longer thread than required: the maximum strength of a steel bolt and nut is when the length of the engagement between the nut and the bolt is the diameter of the bolt. Thus, for a SP10 or SP12 bolt, the max engagement is reached at ½". That bolt has 16 threads per inch, so ½" is about 6 turns of the bolt. Increasing the engagement beyond 6 turns will not strengthen the resistance to pull. Likewise for a 7/16"-14 anchor (SP18), 7 turns will guarantee maximum strength. For our 5/8"-11 SP58 anchors, 7 turns are the minimum to obtain max strength. That said, you can still use longer bolts because the anchors are hollow, and as long as your bolt does not reach the bottom of the anchor, but there are some downsides.
There are drawbacks to using longer bolts in the anchors. One is the amount of time it takes to tighten the extra long bolt. Another is that the bolt thread that is not engaged with the internal anchor thread is vulnerable to rust, and will make it harder to remove the bolt when required. We therefore recommend not using more than 1" extra length when calculating the length of the bolt.
You calculate the length of the bolt by adding the length of the thread inside the anchor (½" to 1") to the length of the bolt above ground. If you are securing a ¼" plate and using our 1/16" thick washer under the bolt head, using our supplied 1" (SP10, SP12) or our 1.25" bolt (SP18) will be perfectly adequate.