At what eGFR do you typically refer for vein mapping for a patient with advanced CKD who prefers hemodialysis when indicated?

This is a big "it depends". Depends on trajectory of GFR loss, likelihood of preemptive transplant, my best clinical guess of the likelihood of successful fistula vs need for graft, etc. But in general, if it seems like HD start would be within 4-6 months.

I don't just depend on the eGFR but also consider the rate of drop in eGFR and the degree of proteinuria. A rough guideline should be around 6 months before we expect them to be on dialysis. Even though I normally don't do it but KFRE equation is also another way of determining when to refer.