The very next day, if you can convince the border agent that you're using your visa for the appropriate means, and aren't trying to 'live' there instead.
They're going to wonder what took 141 days and now needs more. If you're backpacking across the country, say, they can see evidence of your travels and you can show that say, you were now heading up to Churchill to see the polar bears and therefore need another few weeks. They're going to want to know how long you're going for this time, and what you're planning on doing.
It gets a bit more unclear if you're a digital nomad like yourself. Technically you're working for a US company from Canada. Some countries and guards can be very iffy on that. Essentially, they may see it as you living and earning money there, benefiting from their roads and so on, without paying and Canadian taxes. And that's not approved.
However, if you provide them with your business address and show that you're travelling as a tourist as well, it might be deemed more acceptable.
Short version: You're going to have to convince the border agent that you're not living there tax-free, but are a bona-fide visitor, there for normal visitor reasons, who plans on returning to the US where you live.
EDIT: Indeed, from the Canadian Immigration website:
To visit Canada, you must:
- have a valid travel document, such as a passport,
- be in good health,
- convince an immigration officer that you have ties—such as a job, home, financial assets or family—that will take you back to your home
- convince an immigration officer that you will leave Canada at the end of your visit, and
- have enough money for your stay. (The amount of money you will need can vary. it depends on things like how long you will stay, and
whether you will stay in a hotel or with friends or relatives.)
I've highlighted the key bit. If it's apparent or seems that your plan is just to keep re-entering Canada, they may well disqualify you. Indeed, when I lived in Vancouver I had a friend from Utah who went to renew her visa, and was seen to be 'living' there (She was actually trying to move to Vancouver long term, just did things in the wrong order) and was given 2 days to pack up and leave. So basically, that's what your entry request will be tested against.