I have a feeling the final trailer won't drop until the end of March/beginning of April. Just wait until the marketing for BVS dies down before the final push. Maybe an extended version of the super bowl trailer might premiere between now and then. Originally I thought they would reveal Spidey when the merchandise started to roll out but now it seems very obvious that Spidey is not included in pretty much any merchandise run, probably will be in the second wave that releases after the film premieres. Remember, the Vison's look was first reveal from leak concept art from sideshow collectibles.
I feel like Marvel is keeping Spidey under wraps so that the marketing focuses on the heart of the story. Namely Cap, Stark, and Bucky. In the trailers you want to make sure the general audience knows what it's about and why they should watch it. Having Spider-man has the potential to overshadow everything else in the film, being the number one thing people talk about. Also with the baggage of this being the third Spider-man reboot. It could negatively effect people perception of the film and what it is about. So they don't want to over sell Spidey from the get go.
I would say it's a safe bet that we'll first see Spidey in a quick shot in the final trailer and the last bit of promotional material, so probably sometime in April. I'm actually pretty glad they've held back and would actually love to first see Spidey enter the MCU on the big screen before anywhere else. In this time where internet lets everything leaks, it would be great to have that surprise waiting for me. Doubt it will happen, specially since the international release date is different, so I'm fairly certain we'll see the costume before the film premieres. Still would be nice.