For the first few months I always allowed at least half an hour - sometimes she was bored before that but usually that's about as long as she was happy and still exploring/eating, sometimes up to 45 mins. If I gave her things slowly, not too much or too many different things at a time, she would last longer, otherwise she sometimes got overwhelmed early on and acted like she was done, when really it was just a bit too much at once. We always ate together, so it was good for me (and DH when he was there) to sit and take our time!
Now she's a lot more capable of just eating what's in front of her, so maybe 20 mins. If she's not interested in actually eating she lets me know long before that! So now if she starts throwing or pushing food away, we wrap it up.
Suzanne: at 8m my LO was really starting to have a go at most things - pasta was a favourite; rice with some sort of sauce, cottage pie, etc....just put stuff in a pile in front of her and let her go for it! Have you introduced meat? Mine loved big chunks of roast beef to chew/gum on, as well as home-made sausages (from minced pork/beef/chicken) and fish cakes. Vege fritters were good too - eg, grated carrot, sweet potato, or pumpkin. If you don't have any allergy issues, I'd just start giving her whatever you're having, without the salt. (I cook everything salt-free now, and DH and I add it to ours at the table).