How to Check Duration Between Last Modifications?

Hmm, if you don't need historical data, you could try:

You'd need:
1. A "Date" field called "Previous last modified time" or something
2. A "Last modified time" field
3. A formula field that will calculate the difference between the fields from step 1 and step 2
4. A number (or duration) field that will hold the duration value like you said called "Total duration" or something
5. A formula field that will sum the values from the fields from step 3 and step 4, called "New duration"
6. An automation that will run whenever the "Last modified time" field is updated, and its action will be to:
  - Update the triggering record by:
    - Pasting the value from the "Last modified time" field into the "Previously last modified time" field
    - Pasting the value from the "New duration" field into the "Total duration" field

If you need the historical data, you'd need another setup that would create one record per update etc.

One of the problems with your situation is that each key press is consider to be an “update” by Airtable. But you really want to know when the number as a whole was updated. 

At a minimum, you will need some way of indicating when data entry is over (either a button or another field), and a field for storing the previous last modified time. 

How long are the durations? Minutes? Hours? How many times will the field be updated? Do you need historical information?

If you want historic info, consider using a linked record field for data entry. When the value should be updated, instead of directly typing in that field, create a new linked record. Your existing field would then roll up the most recent value. 

Are your users in the grid view, and interface, or a portal? That can affect things too.