We originally planned to visit Montserra on our own by train but quickly realized that was too ambitious given our broken Spanish. So we found a travel agency near our hotel and booked a 5.5hr bus tour.
The tour turned out to be a poor experience - it was advertised to be 5.5 hr in total but 2.5-3hr was spent on getting there and coming back (incl. waiting for everyone). Another 0.5hr on guide explaining things and walking around a bit . So you only have 2-2.5hr left to explore on your own. We were on an English/Spanish mixed tour which meant the guide explaining parttook even longer than usual so we had less than 2 hrs left to explore (11:40-1:20).
We spent about an hour just waiting in line to see the famous black Madona (it supposed to bring good luck if you are trying to have a baby ;) . After that, we decided to skip seeing the boys choir in the abbey at 1pm and went on San Joan funicular at 1250. Took a few shots on top and then quickly came down in funicular ... rushed back to bus. We felt it was a place where we could easily spend 4-5 hours exploring/hiking. If all you want to see is the black Madona and the main bascilica, then a bus tour like this would make sense.