18 month later I know the answer to this question based on own experience (and 20+ shops online).
We use 2 Redis instances - for sessions and for cache (not full page cache).
The reason for that is to avoid session evictions because of the growth of cache data.
For sessions, we assign 100M and no evictions policy and this was never a problem even with long-lasting sessions.
For cache in most cases, 300M is enough for rather small shops to 800M for 15k products. We have one shop with 2M of products and for this shop max memory for Redis is set to 1400M end eviction policy to
allkeys-lru. This shop is working stable for more than 2 years.
We never spotted a problem with
allkeys-lru eviction policy.