Yes, you'd be correct. The agreement (sort of a "morals clause") was in force so long as she was working for Spencer's. As it turned out, she wound up breaking one of Spencer's informal rules on models but in a way that had nothing to do with morals. According to Crystal, Spencer's had a strict limit on calendars with specific models and they were only going to carry a given model for 5 years and no longer; she did 6 years with them.
Actually, she would have done a seventh but she ran into a scheduling conflict with the Spencer's photographers. (IIRC, they shot the new calendars nlt late March of a given year.) This conflict prevented the shoot and, once the streak was broken, Spencers was not interested in shooting the calendar (the Double D Twins, although "both" were actually the one and only Crystal) any longer.
If you should ever bring this up with Crystal, be sure to sort of duck after mentioning it; it is kind of a sore spot with her. Not only did she dislike having to stop shooting the calendar, what really continues to bother her about this was that she didn't negotiate the title of the calendar with her own name -- Crystal Storm -- instead of the Double D Twins. She figured that had she done so, her publicity would have been that much greater.