The "loop" is not really a continuous marked walking track all around. The individual sections are long and some need to be connected by buses or truck taxis. You can take a bus to Zumbahua from Latacunga, but it's not nice a place as Chugchilan, which is wonderful. You can take a bus to Chugchilan from Zumbahua or Sigchos. You can walk from Chugchilan to Isinlivi, but it is long and difficult (we were told, we didn't do it), or from Chugchilan to Sigchos or from Chugchilan to Quilotoa (not much of a town, but the crater is wonderful). We got a ride from Chugchilan to Quilotoa and walked back, about 4.5 hours, but we are over 70 and a little slow. There are some steep downhills. We talked to people who walked from Isinlivi to Chugchilan, stayed over 2 nights, and then walked on to Sigchos.. So how long it takes depends on how much you want to do and which route you take. We had rain one day in the morning and it sometimes rains at night, not very predictably. We were there in December.
We stayed in Chugchilan 4 nights and did mostly shorter walks, all very nice. We stayed at Black Sheep Inn, which was great, but people also like Mama Hilda's, which is less expensive. Any of the places to stay can also arrange transport for you. Some of the bus times are a little inconvenient.