import json
import time
import requests


def get_bets(slug):
    """Return bets on a Manifold market, given its slug"""
    url = "https://manifold.markets/api/v0/bets"
    bets = []
    while True:
        params = {'contractSlug': slug, 'limit': 1000}
        if bets:
            params['before'] = bets[-1]['id']
        new_bets = json.loads(requests.get(url, params=params, timeout=30).text)
        bets += new_bets
        # print(f"got {len(bets)} bets")
        if not new_bets:
            return bets

SLUG = "resolves-the-first-day-there-was-no"

all_bets = get_bets(SLUG)

# Sort ascending by created time
all_bets.sort(key=lambda bet: bet['createdTime'])

# Get the first bet from each unique trader:
traders = set()
bets_from_unique_traders = []
for bet in all_bets:
    if bet['userId'] not in traders:
        traders.add(bet['userId'])
        bets_from_unique_traders.append(bet)

t_last_unique_trader = bets_from_unique_traders[-1]['createdTime']  # in ms since epoch
t_since_last_unique_trader = time.time() - t_last_unique_trader / 1000

print(f"It has been {t_since_last_unique_trader:.0f} seconds since the last unique trader")

Prints currently, for example:

It has been 3231 seconds since the last unique trader

Requires Python and requests library. Modifications or other instructions on request.