The question is a bit of a puzzle. Ideally, there would be a reference book that lists, by industry, how much time you should invest in prospecting activities. Unfortunately, there’s no reference book.
How much time you invest will depend on the number of prospecting activities you plan, the nature of the activities, and the intended results of the activities.
More importantly, different salespeople have different goals, and these goals will necessitate different amounts of time prospecting. Introducing a new product or opening a new territory may take more time than continuing to cultivate an existing market where you already have exposure.
If your efforts are primarily passive, where you have little if any control of the outcome—direct mail or e-mail for example—you will likely have to do more and it will take longer to see results. If your efforts are more proactive, where you have considerable control—cold prospecting or generating and then calling on referrals for instance—you can invest less time.
Since there is no simple formula, you must consider your prospecting objectives and then carefully track your activities and results. Then, you can decide how much time you want to invest and choose the activities that will allow you to achieve your objectives in that time period.