Frequent confession is, naturally, a desirable activity. It is holy and edifying for our spiritual growth, and in particular, we need spiritual advice from a learned father of the soul (aka our priest). Spiritual direction seems to have a stronger emphasis in Eastern Orthodoxy and Eastern Catholicism. They (rightly to mind) do not regard the Sacrament of Renconciliation as being solely about confessing what sins one has committed but rather the opportunity to discuss one's progress in the spiritual life with a father who can help us take further steps along the path.
Oddly enough, one will find mystics such as Saint Catherine of Genoa who - contrary to the majority - hardly went to confession. I wouldn't advise this though - Catherine had an incredible interior state of purgation which is unique in the history of Christianity, and because of this particular grace, she did not need confession as much as most people (myself included do).
In sum: Go as often as you feel the need. If something is troubling you, if you feel the weight of attachments, cravings, inordinate desires, errors of judgement or bad choices upon your soul then go and receive absolution, penance and guidance from the Holy Spirit through the minister of the sacrament.
Personally I wouldn't go only if you have committed "mortal sin", as if some sins are less in need of help, even so-called "small ones" can trickle to become larger issues blocking your path to holiness. That's one of the reasons why I have always had a problem with the Latin distinction between mortal and venial. Yes its good for scrupulous people but...hmmm...
Also, only one's spiritual director can truly judge the gravity of a sin for a particular person (ie we shouldn't self-diagnose, if we think something is severe enough to hurt our relationship with God then go and get help and guidance from a priest).
Read this by Saint John of the Cross:
"...If one small crack in a pitcher goes unrepaired, the damage will be enough to cause all the liquid to leak out. ... Accordingly, one imperfection leads to another, and these to still more. You will scarcely ever find a person negligent in the conquering of one appetite who will not have many others flowing from the identical weakness and imperfection caused by this one appetite. Such persons, consequently, are ever faltering along the road. We have witnessed many persons, whom God was favoring with much progress in detachment and freedom, fall from happiness and stability in their spiritual exercises and end up losing everything merely because they began to indulge in some slight attachment to conversation and friendship under the appearance of good. For by this attachment they gradually emptied themselves of holy solitude and the spirit and joy of God. All this happened because they did not put a stop to their initial satisfaction and sensitive pleasure, and preserve themselves for God in solitude..."
**- Saint John of the Cross( 1542 – 1591),
(Ascent to Mount Carmel, Book I, Chapter 11). **
Even when the fault is not big, it still serves to block us from deeper growth and deeper happiness. John has an axiom that says it doesn't matter in the end whether a bird is attached to the ground by a heavy chain or a light string - it can't fly in either case. Hence, he cautions us strongly against being comfortable with any of our faults or addictions by rationalizing that this or that fault is not so serious and that we are fundamentally good persons, despite our weakness. Whether we are held by a heavy chain or a thin thread, we still can't fly.
If we grow comfortable with an addiction or fault inside us, we will find ourselves impoverished too in another way: it will rob us of real happiness.
I view sin as an illness. If you feel "unwell", unwholesome and as if something is impairing you spiritually then I would go to the soul doctor - confession, even if you cannot pin-point a specific sin that is bothering you. Its your overall mental/spiritual condition that I see as more important anyway.