Is it correct that there is a period between Tashahhud and Salam during which Dua is accepted? If so, is it allowed to do a lengthy Dua of around 5 minutes at this time or is it more preferable to keep it brief?
There is no specific period between Tashahhud and Salaam as both should be recited simultaneously. Du'a is always permissible during any part of Salah including Tashahhud and Salaam or between them. We have narrated supplications with Tashahhud. The Du'a can be short or long but it should not take you away from the Salah situation.
'Wassalam.