Well if you allow for media files from the respondents, storage is actually an issue.
If you would like to avoid buying additional storage, I recommend going offline with the questionnaire during the night or in the early morning, when nobody is filling out the questionnaire. Then download all (!) data, make sure that your data can be opened, make a secure backup and another one (on a different location!). Then delete the data collected so far and go online, again. Please note that - othen the records - single media files can not yet be deleted.
Depending on how you recruit the respondents, it is useful to make such a break before sending or posting a reminder.