Usually 15-20 minutes on luggage either way. But the return trip involves customs, so you're not going to see your luggage until after going through the line which can be a class 2 hurricane or a small shower (better allow 2 hrs for total SJU time). Each time we go to Tortola via SJU > we decide to go via STT the next time. Then, we get upset with Ferry system at St Thomas and decide to go via SJU the next time. If you haven't purchased tickets yet, you might look at going one way STT and SJU the other. We have done this multiple times depending on price/arrival/departure needs.
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