Right now, there isn't an auto-save feature within QuickBooks Online. It would definitely help to be more efficient in working with invoices or estimates. I'll send in a feature request to our Product Development team about this.
In the meantime, to make sure you won't lose any data while working, the best practice is to periodically click (or use the keyboard shortcuts) Save (ALT + S) or Save and Close (CTRL + ALT + D).
Another workaround you can try is to set the time limit to a maximum before QuickBooks signs you out. Here's how you can get that done:
1. Click the Gear icon and select Account and Settings.
2. Click Advanced from the left panel, scroll down to Other Preferences, and click the pencil icon.
3. In the Sign me out if inactivefor section, set it to 3 hours.
4. Click Save and then Done.
If you have other questions we can help you with, feel free to swing by the Community anytime. See you!