Answer: It depends!
It depends on if you are talking about inbound or outbound telesales; how long a sales transaction takes and how much after sales processing is needed.
For comparison you should have some sort of model or benchmark transaction that can be used to assess the talking time and other time that a telesales rep. spends on a sale. I.E the average handling time.
For Inbound sales, the percentage of time spent talking on the phone will also depend on the occupancy of the sales ACD group. This will depend on the call volume, call handling time, the number of staff on duty and if they have been sized to answer at an expected service level.
With long call durations and low call volumes, occupancy, even at the optimum staff size can be very low. For example you might need 3 staff to answer calls that take an average of 15 minutes to handle, even if you only got one per hour, but wanted to be sure to answer 99% within 30 seconds, because there is a reasonable chance that you will have 3 concurrent calls. Mind you, if your calls only took 15 seconds to handle, but you get 60 per hour, 1 person may be more than enough, because the calls are very short, and you can give 2 answers every 30 seconds.