Although we do not usually think of homilies as an occasion for liturgical abuse, homilies that are extra long can in fact be considered a type of abuse. While there is no strict guidelines on this, we do have some pretty authoritative statements on this from the then head of the CDW Malcolm Ranjith as well as the Secretary General for the Synod of Bishops. Both suggest short homilies, 8-15 minutes maximum.
Archbishop Malcolm Ranjith, Secretary of the Congregation for Divine Worship, stated that 10 to 15 minutes was sufficient length for a homily and suggested that homilies longer than this indicate “scarce preparation” and goes so far as to say that they are a type of liturgical abuse. Here are his comments when asked about long homilies:
"This also is an abuse…As for the homilies, they must relate, as the Pope has underlined, exclusively to the catechetical aspect, avoiding sociologisms and useless chatter. For example, priests often veer towards politics because they have not prepared well the homily, which must,instead, be scrupulously studied. An excessively long homily is synonymous with a scarce preparation: the correct time for a sermon must be of 10 minutes, 15 at most. It must be acknowledged that the culminating moment of the celebration is the Eucharistic mystery, which does not mean downplaying the Liturgy of the Word, but clarifying how the liturgy must be applied." 
Another opinion: Bishop Nikola Eterović, Secretary General for the Synod of Bishops at the Roman Curia, suggests that the homily ought to be no longer than eight minutes, which he bases on the average time a person is capable of concentrating. In his book, Word of God, he writes:
“It is useful to remember that in general the homily should not be longer than eight minutes, the average time listeners can concentrate. The preacher can write the homily, but at the time of delivery he should use an outline, a special guide that will allow him to follow a logical line of thought while looking at the faithful…Improvisations must be avoided, since the homily is too serious of a reality to be delivered to the faithful without adequate planning.” 
While neither of these statements is authoritative in and of itself, these words from these two very important bishops give us some insight into the proper pastoral approach to homily length. The average homily of the pope is also in the ballpark of ten minutes or less, which is another indication that homilies longer than 10 to 15 minutes are excessive in length and should be curbed.