There's not necessarily a hard limit, it's just whenever she gets bored, which is usually when she's nearing complete immobility. The mechanism of her growth is kind of weird and complicated because it's technically about praise, but it's also based on her own perception of praise. She defines basically anything drawing attention to her size, muscularity, unnatural ability to grow, etc. as praise and will interpret possible negative modifiers like freakish, grotesque, inhuman, etc. regarding her body as praise and use that as fuel as well. In addition the praise can also be entirely imagined on her part, ie if someone is looking at her she can completely fabricate in her own mind what they must be thinking of her and use that. As long as there is someone else around to witness her she can basically talk herself into getting bigger.