In my opinion, inside sales generally don't require much experience.. An experience for few couples of months is enough for a person to be successful in inside sales.. But the skills it requires includes:
1) Smart Work
2) Excellent oral & verbal communication
3) Thirst to achieve desired targets
4) Consistency
5) Knowledge of product and how the prospective customers can take maximum benefit of it.

And if a person has experience but doesn't possess these skills, he occupies no space for Inside Sales on the other hand if a person has these skills, with experience of about 1-3 years, he could be the smartest person in any organisation which promotes inside sales..

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As rightly said by deepak its the skills that make someone successuful & not the experience irrespective of its inside sales or field. In fact sales is field where at the end of the day its the achievement of the targets that matter than anything else. So if a sales person with just few years of experience has the skills of building relationship with the prospects, has the product knowledge & understand his clients needs than he can be more successful than person with more experience but lacks in some skills. Hence one should constantly strive to get perfect in sales skills to become successful in area.

Only experience doesn't matter to become a successful sales person, To become successful in sales whether it is inside or field you should always enhance your knowledge, skills, qualities with your experience. That is the reason why sometime people having experience of 8,9,10 years remain at the same level while people with lesser experience reach the top level.

Along with experience you should work on below mentioned qualities to become successful in inside sales:

Time Management
Calm & Competitive
Dedication & Discipline
Strategic Planning
Pitch Perfect
Professional Team Player
Exceptional Writing Skills

How much experience a person should have to be succesful in Inside Sales..??