Deriving life expectancy from FLIPI index data for FLIPI(3) High Risk Deriving life expectancy from FLIPI index data for FLIPI(3) High Risk
I completely rewrote my very terribly simplistic initial question (currently erased) to take into account two key elements of advice from the original answer. (1) Median years of survival is a good measure of life expectancy for patients with typical disease progression within a FLIPI score. (2) Survival rates have drastically improved in the Rituximab era.

The above is from Figure 4 from this paper:
Follicular Lymphoma International Prognostic Index 2004
The 6.16 years median life expectancy referenced by the answer below may be a very reliable measure of typical outcomes for those with typical FLIPI(3) disease progression.
Based on the advice in the answer I found a sufficiently large sample that recalibrated the overall survival rates for the Rituximab era:
Follicular lymphoma in the modern era: survival, treatment outcomes, and identification of high-risk subgroups 2020

Because the number at risk for 10 years had 58% of the
subjects drop out of the study we may not be able to trust the graph's 69% survival rate. Only 9% of the subjects dropped out at the 5 year mark so we can probably trust its 82% survival rate. This is a 55% improvement over the original FLIPI data 53%. The graph uses Kaplan-Meier to adjust for missing data.
When we simply multiply the above 6.18 year life expectancy by the 55% increased life expectancy provided by the Rituximab era we derive a 9.56 year life expectancy for patients with a FLIPI index score of 3 (eventually treated with Rituximab or equivalent) having typical disease progression.

The original answer was based on a very simplistic analysis of 53% five year overall-survival and 35% ten year overall-survival.

Those calculations aren't correct. You can't reliably turn
overall-survival percentages at any given year into a survival
estimate at other times unless you know the shape of the survival
curve over time and make certain assumptions. EdM

I took two key aspects of this original answer to derive an updated analysis:
(1) My analysis changed its basis from 53% five year overall-survival and 35% ten year overall-survival to median years of survival. Median years of survival is a good measure of life expectancy for patients with typical disease progression. For patients with a FLIPI 3 score (based on the original FLIPI data) the median years of survival was 6.18 years.

(2) The above estimate can be made much more accurate when considering data from the Rituximab era. (shown above).
The 69% ten year survival is based on 58% of the subjects dropping out of the study. The median 13 years of survival is based on 98% of the subjects dropping out of the study. Because of this we will extrapolate median survival rate based on the  82% five year survival rate where only 9% of the participants dropped out.
A reasonable estimate of life expectancy for patients with a FLIPI 3 score (based on data from the Rituximab era) and typical disease progression is a 55% increase over the prior estimate: 6.18 * (82/53) = 9.56 years. (82% was interpolated directly from the graph pixels).
Patients with a FLIPI index score of 3 (eventually treated with Rituximab or equivalent) having typical disease progression have a life expectancy of 9.56 years.
