Elemental Oath lasts for 15 seconds after a crit. Lava burst has an 8 second cd so on the odd chance that both your shammies miss with it, in another 8 seconds you can cast again (so 8 seconds plus 1.5 second cast - both miss - 8 seconds plus 1.5 second cast - EO back up again) 19 seconds after the buff cd starts to when it gets refreshed so that's 4 seconds that EO won't be up, which you reduce to 2.5 seconds because 1.5 seconds is used casting the lvb after the miss which you know is going to crit. I didn't include travel time because the travel time at the end is cancelled out by the travel time of the first cast. And I didn't consider haste, which means 2.5 seconds is a worse case scenario.

Now, if you happen to miss with both lvbs both times, well, it's just not your day and 2% crit isn't going to do anything for you haha.