One unknown in that sort of calculation is whether the ~3.5 billion years between the emergence of unicellular life and the emergence of complex life forms in the Cambrian explosion is typical of what it would take somewhere else.

It's possible that the transition took some sort of triggering event, whether a big environmental change like Snowball Earth, or some sort of random evoluntionary development, that could have happened much earlier, or much later. It does seem like the pace of change until that point was awfully slow, though of course some important things did happen, like converting to an oxygenated atmosphere.

With our present knowledge, it's hard to know whether ~4 Ga to complex life is typical, really fast, or downright retarded.