In January, for potential weather delays, what is a safe layover time at ORD?

Five and half hours are more than enough for any half way reasonable operating condition. However, in January its possible that you will encounter outlier conditions such as a major snow storm. There is no guarantee against that, and no safety margin will be truly "safe", even if you fly in the day before. While it's not likely to be that bad, it can happen.

If you are really worried about this scenario, your best bet is to book everything on a single ticket, even if it's a little more expensive. The main advantage would be that the airline could route you through a different hub from Seattle if Chicago turns into a mess. If you were to fly with United, they could route you through Newark, Washington, San Francisco, Detroit etc. Even if you get stuck in Chicago, the airline would eventually get you through to your destination at no additional cost.

If you miss your flight in Chicago, you can plead with the connecting airline, but chances are they will charge you at least a change fee. Another thing to consider: if your flight from Chicago is delayed or moved to the next day, you'd have to rebook or change your first flight from Seattle to Chicago too.

A little research shows that, yes, January is the snowiest month (sort of like asking if one should expect rain in Seattle) with O'Hare Airport having 28.5 snowy days a year.

In January, it snows, on average, 8 days during the month, for a total of just under 11 inches/27 centimetres. Most of those days, the snowfall is about 1 inch (2.5 cm), occasionally 3 inches (7.6 cm) and, very rarely, 5 inches (5.7 cm) or greater.

That being said, Hopper Research reported O'Hare as the worst for winter delays:

• Avoid O’Hare in Chicago! 42% of flights are delayed at O’Hare during the winter
• Chicago, Newark, Denver, and Fort Lauderdale have more than a third of flights delayed during winter months
• Airports in warm areas like Florida can also be impacted by winter storm delays in other places, due to their popularity during the season
• If you’re flying out of New York, you’re less likely to be delayed if you fly out of JFK or LaGuardia instead of Newark

I've seen 5 hours delay at ORD once, however I think your odds of success are extremely good on a random January day with almost 5 hours to spare.

Remember if the incoming flights are delayed the outdoing ones are usually delayed similarly- especially at that time of day the flight will likely have arrived from somewhere else not that long before departure.