Chiku has been with me since September and she still has the occasional bit of soft food from me.. though it is getting less and less.. They will decide when they don't want you to give them anymore so just keep offering it till they refuse.
It's also a good way of getting stuff into them as KThorton said - if you ever need to administer meds (50 years with never having to do this would be rather lucky!!) Or you could mix the warmed food with Red Palm Oil (I found some in Asda), that will help keep feathers and bird in tip top condition.. Hell, even whack some Nutrobal or Calcivet (which ever supplement you use) in there, every little helps.
I used kaytee's handfeeding formula when I got Chiku home.. Sadly i didn't know about that stuff when I got Echo - so she was brought up on mashed banana but seems none the worse off for it.