There is no minimum usage time, but the longer you use it, the greater the benefit and profit you can make from the technology. The benefit is proportional to the time of use.
hey!U is aimed at teachers who are new to the use of technology in the classroom, so they could even use it without even having Internet in the classroom, benefiting from part of our technology. I wouldn't need any previous preparation of the class, or any kind of computer knowledge, the only thing I would need is for the students to use the devices in the classroom. If on the contrary we use hey!C, what we could need is a previous preparation of the class, with questionnaires, exams...it is focused to more technological teachers, and that they want to give one more turn to our technology not only obtaining visual feedback, but also obtaining detailed reports of the class, of each pupil....
They would be sent a user guide, so that they could see step by step how the tool would be used, but in any case we think it would be interesting an online training via skype ... for them to see it and resolve possible doubts.