An Icelandic scientist says that aircraft shouldn't overfly the volcano Hekla, as it's numerous eruptions occur with only 23 to 79 minutes warning.
Link to article (in icelandic)
Varasamt að fljúga yfir Heklu - mbl.is
He lists the numerous aircraft that have overflown the volcano, only today.
He says it only takes the ash cloud between 5 and 20 minutes to reach cruising levels.