I would like to understand how far we need to hold on in the loss phase. What are the key indicators during the loss phase that we should stop now or keep going.
Opportunity cost is by far the most expensive component for me. The time spent on my SaaS product would otherwise be sold as professional services. These cumulative lost billings are orders of magnitude more than I've spent on hosting, domain name, etc. It's important not to lose sight of this.
That being said... You don't need to spend a single dollar in the "loss phase"! Validate your idea upfront. Talk to potential customers. Look for a first iteration which delivers value without incurring out-of-pocket costs to you.