How long will Altera support AHDL?

I doubt it - check out any of the db directories - they are full of AHDL.

I ask because Altera seems to follow the majority and I believe the majority is using Verilog and VHDL. Altera's newer megafunctions are now being written in SystemVerilog. Maybe Quartus III will be free of AHDL.

Altera havent done anything with for at least 10 years, and discouraged its use for at least that time.