As with determining an initial budget (see "How much should I spend on my campaign"), many factors go into setting an ongoing budget for a campaign you'd like to be "evergreen" (that is, always on). We typically spend 4-8 weeks scaling a budget to an optimal level and finding efficiencies. It's important to consider seasonality for that budget, that is, times of the year when you may want to increase spend to reach demand, or times when media is more expensive because demand is down. (For example, it makes more sense to increase the advertising budget for a Christmas Boutique in November and December, than it is in July.)