This is where I think there could be much disappointment. On the basis of this verse in particular, many expect the Jerusalem temple to be rebuilt. However, what if this does not refer to the Jerusalem temple? And there are several factors against it:
First, word usage. The word 'temple' appears about 15x in Revelation. Twice, in ch 21, it does not refer to any particular temple. The remaining 12 other times refer to the temple in heaven. If this is the only reference to the Jerusalem temple, one would expect John to say so, given that it would be an exception rather than the rule. But he does not. Consistency of word usage favours this to also be the temple in heaven.
Second, counting of worshippers. The word used for temple in 11:1 is the sanctuary itself, i.e. the holy place, not the whole temple precinct. Only priests enter the holy place to perform their duties and even then I think only at set times, e.g. Zechariah in Luke 1:9. The common Jew worships in the courtyard outside. Unless the rules change for the end time temple - and there is no reason to expect change - there would hardly be anyone in the sanctuary of such an end-time temple for John to count. On the other hand, all believers are allowed to worship in the temple sanctuary in heaven, and so there would be people there for John to count.
Third, the way it is introduced. If this is an end-time Jerusalem temple, it must be a vision because the temple had already been destroyed by the time John wrote. John often, though not always, introduces a new vision by "And I saw," e.g. most recently in 10:1. However, the way this temple is introduced in 11:1 is as if John finds himself in the vicinity of the temple. And by the end of the chapter, in vs 19, John is looking at the ark inside the temple in heaven. Would it not be more natural to think that what he measured in vs 1 was what he was looking inside later in vs 19?
I am not saying that the Jerusalem temple will not be rebuilt. Man is able to do many things but they need not be part of biblical prophecy. However, if 11:1 does not refer to the Jerusalem temple, there is nothing that requires the temple to be rebuilt. Then the risk is that the end could come unexpectedly if we rely on such signs to prepare ourselves for the end time.