How long will my plugin data be kept after deactivation?

How long will my plugin data be kept after deactivation?

As long as the date on which you clicked the ‘deactivate’ button is BEFORE the current subscription renewal date, the data pertaining to this plugin will be retained. After that the data pertaining to this plugin is deleted, the only exception being the payment processor plugins. The data for these plugins typically comprises the reference numbers and auth numbers for the credit card transactions done when the plugin was active, and that is why this data is retained for reporting purposes. This data ( reference numbers and auth numbers) is never deleted. 
Plugins in Tagrain are either Subscription Based or License Based. So the price that you pay for your plugins is based on the number of POS terminals that you have in your setup and the number of licences that you have acquired.
There will be no change in your subscription amount in the current billing cycle if you are only deactivating a plugin AND not updating your Store / POS count. 
Once activated, you cannot deactivate the Multiple Units of Measure plugin because deactivating the plugin would affect the entire master data and all the transactions across the application leading to lead to rounding off discrepancies.