Not enough front density after FUE HT. How long can I wait for second HT? (photos)

It appears from the photos that you have very fine hair.  The creation of density with your hair caliper is difficult to achieve with one procedure.  Fine hair lends to very natural-appearing results after hair transplant but it also creates a challenge providing density.  1 year after a procedure is adequate to have a secondary procedure to further fill in the recipient area.  You may also want to consider low level laser light therapy to help slow down the progression of hair loss and help to thicken miniaturized hairs.  

Jack - From the picture where you pull your hair up, it looks as though a fair number of what is left there are tranplants.  After a year, you certainly can be ready for another session of HT.  Being in your late 20s, it would be important to get a good family history of hair loss.  You may also benefit from compounded minoxidil products.  Best talk to your hair restoration surgeon about all the options that you have as you certainly may have more.  Best of luck.