This is kind of an odd question because they can sit unassisted when they can sit unassisted, kwim? There's no definitive timeframe and it also depends on what you consider "unassisted." They can balance for a few seconds long before they are truly sitting (i.e. playing & looking around without toppling) unassisted. DS2 is far from that stage now but with DS1 I recall having family pictures done when he was 6 months old and he still wasn't at the stage where he could be trusted not to topple during the pictures.
Yeah, it depends how you define it. DS2 will sit unassisted for quite a long time now, but I still make sure I'm around or he has a boppy around him since he will inevitably topple eventually. He'll be 6 months tomorrow. He started tripodding at about 5 months and just kind of got better a little at a time.
My daughter is about 5 1/2 months (24wks) and she can sit on her own but I stay close or put the boppy pillow behind her cause she still flops over from time to time.
Technically DD can sit unassisted (5.5 months right now). However, she hasn't sat up on her own yet. I'll sit her on her play mat in front of a toy and she'll stay there for a while, but if something behind her grabs her attention she'll topple over trying to see what it is. She's trying to sit-up from a lying position on her own but hasn't figured it out yet.
Re: Sitting unassisted