This is a great question! I'd also like to hear the best answer to this. One choice is to create a new file each time. This would be good if there were many minutes between sessions of sending data. Another choice is to keep the file open and continue to send data every minute or less. This is great if there is data available to send every minute. There is a parameter to choose the timeout length. The downside is power consumption. I think we need more information to answer your question.
I will create another thread for my FTP question.