The minimum time under artificial UVB is about two hours per day, seven days per week. When she spends time outside, as long as the sun is high enough in the sky, 3-4 hours a week has been reported to be enough, but I would go for 5 or six hours/week under full midday sun if she's getting no artificial UVB.
Many owners set up heat and UVB where their iguana loves to hang out. If the back of the sofa is warm enough for a period of time during the day, you can throw a large towel over it to protect it from UV, and set up another UVB there that is close enough to the skin on her back. What you want is to be 'sure' she gets enough UVB time.