The latest Chronicles of Elyria design journal has been posted to the official site that seeks to assuage fan concerns about how spirit walking affects play time and much more. Called "The Weight & Measure of a Lifetime", the diary is divided into several categories that include the length of an Elyrian life, how fame affects the death toll, and others.
Of particular interest is the section about spirit loss, beginning with the base spirit loss for a death requires spirit walking for two real-world days.
Now, let's assume you die a bit more frequently, maybe once per week. Each death would shave off the same 2 days of play time. As a result, every 7 days would actually count as 9 days of play time. If you divide the average of 354 by 9, you get approx. 39 weeks. So if you're someone who dies an average of once per week, you can expect to live approximately 8.75 months without needing to buy a new Spark of Life.
Continuing on with the same model, if you died an average of 2 or 3 times per week, you could expect to live for either 7.25 months, or 6 months (rounded to the nearest week) respectively.
Now let's assume you die very frequently. You're out adventuring every day doing extremely dangerous activities. Every day you die once. That means every day counts as three days of play time. The average of 354 divided by 3 is 118 days. That's 16 weeks, or approximately 3.75 months before you need to buy a new Spark of Life.
You can read the comprehensive diary on the Chronicles of Elyria site.