Do you need to deep condition your hair? Can you over-condition hair?
1. Penetration by some ingredients into hair (noting that bleached/damaged hair is known to be able to take up more than undamaged hair). - see here
2. 20-30 minutes is the maximum aDsorption time for conditioner and at a temperature of around 35°C again on damaged hair - see here
So do you need 20-30 minutes and 35°C?
No you do not necessarily need to leave conditioner for that long. The length of time you leave conditioner on your hair is proportional to how damaged your hair is. Remember that conditioner works best on damaged hair as it tends to get stuck in cracks and flaws. The less damaged your hair is, the less need you will have for long conditioning time. If your hair tends to get overly soft, mushy or if your hair has curls that can form spirals/ringlets but tends to not hold a curl with longer conditioning time, then you are in the group where 5-10 minutes is good enough for you.
How can you tell if your hair is damaged if it is not treated in any way (i.e not bleached / not colour treated/ not heat straightened)?
You should regard your hair as damaged or having the potential to become damaged if you have a problem with breakage or split ends. If your hair needs supreme care not to snap, regard it as having potential to be damaged. If you constantly see split ends or mid shaft splits, your hair is damaged. In either case, you may benefit from longer conditioning time (20 -30 minutes).
Can you overcondition your hair?
I used to regard this as a nonsense term but when I did some research, I do now know that it is possible. A study done on nails (same keratin protein as hair) was performed to find out why nails get weaker with repeated water exposure. The study found that exposing nails to water for over 15 minutes (remembering that at 15 minutes the protein is saturated) led to the keratin coiling different from normal and this was linked to softening and weakness (BBA,pp 210-216,1999).
There are many naturals who will condition their hair for hours at a time because they like the softness that it develops. If you are in this group, you are someone who likes over-conditioned hair. The softness you are feeling is most likely related to the change in the keratin and you should be careful when handling your hair when it is that soft as it will be weaker until it has time to recover its stronger conformation.