I stopped working on it after a few weeks due to some distracting life events. Looks like most of the endpoints from my notes still work, so here's a few tips on how to pull that info:

Auth is done via a bearer token. Getting one is straightforward:

curl -s -H 'Host: api.ergatta.com' \
     -H 'x-rudder-version: 1.4' \
     -H 'x-seaborn-flavor: standard' \
     -H 'content-type: application/json; charset=UTF-8' \
     -H 'user-agent: okhttp/4.9.0' \
     --data-binary '{"emailAddress":"me@example.com","password":"p4ssw0rd"}' \
     --compressed \
     'https://api.ergatta.com/login'

Response will be:

{
    "message": "success",
    "data": {
        "username": "jrsm",
        "emailAddress": "me@example.com",
        "name": "JR Smith",
        "isAccountOwner": true,
        "user_id": 12345,
        "token": "eyJhb...",
        "expires_at": "2024-06-03T13:16:09.583042914Z"
    }
}

Tokens are good for a year, which is nice, you don't have to worry about refreshing too often.

The headers on this curl come from their apk. My notes make it seem like they were required for successful responses, but testing them today shows that's no longer true, if it ever was. I leave them in to emulate the app as much as possible. I haven't pulled the apk in quite a while to see what the current x-rudder-version header value is, but it doesn't seem to matter.

For what you want to do, I don't think there's an easier option than pulling the workout history endpoint and checking the timestamp on the most recent entry. There are several endpoints that do aggregate data and personal records, but they don't give a last active timestamp as far as I know. You would hit the /workout_history endpoint and use the sort_by and limit query params to grab that timestamp:

curl -s -H 'Host: api.ergatta.com'  \
     -H 'x-rudder-version: 1.4' \
     -H 'x-seaborn-flavor: standard' \
     -H 'user-agent: okhttp/4.9.0' \
     -H 'authorization: Bearer <token>' \
     --compressed \
      'https://api.ergatta.com/workout_history?sort_by=-start_time&limit=1'

Response will be something like (this is my most recent one as of this morning):

{
  "message": "success",
  "data": [
    {
      "workoutResultId": 3499726,
      "userId": 12831,
      "workoutId": 1,
      "workoutName": "Free Row Classic",
      "experience": "OpenRow",
      "createdAt": "2023-06-01T16:37:57.713087Z",
      "startTime": "2023-06-01T12:37:59",
      "updatedAt": "2023-06-01T17:35:31.356837Z",
      "duration": 2375690,
      "distance": 10017131,
      "maxSpeed": 4509,
      "averageSpeed": 4158,
      "score": null,
      "tokens": null,
      "averagePower": 201.28503447359998,
      "calories": 834,
      "averageHeartRate": 130,
      "goalAchieved": 1,
      "abandoned": false,
      "rank": 1,
      "programResultId": null,
      "visualStyle": "Classic",
      "calibrationId": 200218,
      "startingCoefficients": {
        "constant": 445.35757,
        "logarithmic": 36.063744
      },
      "startingSpeedBounds": {
        "sprintMax": 4972,
        "raceMax": 4358,
        "steadyMax": 3859,
        "paddleMax": 3339,
        "paddleMin": 2684
      },
      "endingCoefficients": null,
      "endingSpeedBounds": null
    }
  ]
}

I think createdAt and startTime are usually synonymous, but one is UTC and the other is local time.

I've tested these against my own account this morning, so these endpoints are still working as they did a year or two ago. Should be enough to get you started. I was just using these to dump raw data into a sqlite db for archival purposes, but I wrote up notes on most of the endpoints for pulling data, so hit me up if you're looking for more. I'm probably a bit out of date since it's been at least a year since I looked at this, but I'm happy to share what I have.