There is no fixed limit on the amount of data you can store on your device when in offline mode however this will depend on the available space on the device.
The data will be kept offline, granted that you don't clear the browser's cache, logout or re-sync the data while offline.
Also when offline you must also take into account potential data conflicts, and the longer you remain offline the less likely it will sync correctly. We recommend offline mode is used for intermittent internet connections.
Any template changes by other users that occur while you are offline may cause the offline data you have to be corrupted if it cannot sync straight away.