Your sentences seem to be check-plus work (completely correct); now on to your situation with "waiting."
With this, "to wait" would be too passive--It doesn't take a long time to physically start waiting, which would be what "They don't know how long it takes to wait for a love like this" would imply. Waiting, however, is the physical action of waiting, which is what Jason Mraz means here--That it takes a long time of waiting to find a love like that.
Man, with verbs like "wait" this does get awfully confusing, doesn't it?
Just a note of caution--In general, taking grammar rules from songs isn't the best of ideas, due to their, for a lack of a better word, puffy and artsy phrasing--They're not looking to be direct to the point (or always make much of a point, either!), they're, in general looking to get eventually to their point in the most artsy way possible.
As always, any questions? It's getting pretty late for me, so I may have misphrased/awkwardly phrased some things.
Student or Learner