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There are a few common practices for configuring when backups occur. The most common option is on a fixed schedule, such as once a day, week, or month. The second, which we prefer, is to upload file changes whenever they're changed and saved, otherwise known as a continuous backup setting. Services only transfer the modified part of the file in this scenario, so as not to overburden your internet connection or take up unnecessary storage. A third way is simply to upload files manually. Some may appreciate this degree of control, but this method is only effective if you remember to regularly run the backup.
CrashPlan is completely free if you're just doing local backups, but even online backups are affordable, with CrashPlan+ accounts starting at $2/mo (per computer) for 10GB of online backup storage, and going up to $4/mo (per computer) for unlimited online backup storage and $9/mo for unlimited online backup storage for a whole household. You can check out their plans here, and try them free for 30 days with a new account.