If we know what we are doing, it can help us to develop more self-reliance. The solitude can deepen the practice if, as I said, we know what we are doing. At other times, if we don't know what we are doing and we are not using wise and proper techniques, it can make us continue going in a wrong direction for a long time without any instruction to get us back from that.
Steve and I usually go into personal retreat for a month in October/November, where we guide ourselves in meditation and don't have much contact with anyone for a month. It's great for developing the inner strength and confidence in the Dhamma, as you start to see how you can direct the mind and use the Dhamma to guide you through difficulties and problems and begin to have insight and understanding arising. However, you have to actually have a clear understanding of what to do, because sometimes you are tested in these situations and if you don't know how to balance the mind when it gets into difficulties then the retreat might not have the benefits you are seeking.
We usually do not advise people to go into personal retreat for longer than they have done with a teacher-led retreat. To actually see how you go, say if you've done ten-day retreats with a teacher, then try a ten-day personal retreat. However, don't go into it as intensively as you would if you had a teacher there, because solitude can get you in touch with some of your insecurities. So you have to have a clear understanding of refuge, as far as I understand, refuge in the Triple Gem. Knowing confidence-inspiring methods of meditation is very helpful when you are in a solitary-style meditation retreat, as well as the ability to open to Dukkha. So a compassionate intention and a good practice of compassion/Lovingkindness is very helpful, so you can know when to back off and when to push.