When is the right time for seclusion? When is the wrong time for seclusion? How much time should be spent in seclusion?
I practice with Triratna Buddhists so for us solitary retreats are seen as an important part of practice. So much so that if one wanted to become ordained (as many do) then the expectation is of at least one solitary retreat. So if I can have a go at your questions
When you have been practicing for at least 6 months and have attended at least one group retreat
How much time
Depends. The people that I know usual do a week but many start off with a few days. Ultimately people can and do spend months on solitary
I don't believe I've ever heard of anyone saying a wrong time except with the above caveats. I guess as ever you should kind to yourself on these things. Even though it is solitary it is up to you how you spend the time. It doesn't have to be 8 hours meditation a day if that isn't appropriate for you at that point. Many people for instance have spent some of their time writing (about the dharma) and have found that beneficial for them at that point.
I have never been on solitary retreat so this is all second hand. I do believe it is a valuable practice though so would like to at some point.
Basically: whenever you're ready for it. Solitude is a human right.
Technical Buddhist answer: when you have achieved >= "access concentration", preferably 1st jhana, optimally 4th jhana. Also you should know what what you're going to be doing (vipassana/jhana, how long, etc. read schedule).
If you don't have at least access concentration, you'll just flounder in emotions and what-not until you get access concentration. It's a catch-22.