Interesting thread. I do use wort to carb in my keg. I generally use 2 pints of wort. As far as time to carbonate. My last APA was carbonated in about a week, so it didn't take that long. I brought it to a party about 2 weeks after brewing and most of it was consumed (sad to say). I did have to add some CO2 to get the APA out after a while, but the whole thing was carbonated.
I used to carrb without a spunding valve. I would just take daily samples into a chilled glass and see how fast the pressure was coming out. This was my way of spunding. For my current batch, I have a spunding valve. It looks like most others, but I didn't bother to clamp the hose. I set it at about 35psi. It stayed there for a while and went down to 5psi surprising, but the same as reported above. Then it was up to 10 psi. I didn't have time to check it last night. Our house is quite warm, so I created an expanded pressure/temperature chart. I used a chart from an on-line keg company and expanded the temperature up to 80 degrees F. I used the formula V = (P + 14.695) * ( 0.01821 + 0.090115*exp( -(T-32)/43.11 ) ) - 0.0033. This didn't match perfectly with the keg table, but it was close. I tried to upload the chart, but the message said, Your file of 254.3 KB bytes exceeds the forum's limit of 19.5 KB for this filetype. That would be a tiny file!
At any rate, the chart shows at a temp of 78 degrees, 2.3 volumes of CO2 would be about 32 psi. This would be the pressure I need. The trick now is to get the correct setting on my pressure relief valve. It seemed like when I was setting it up the actual pressure was 35 psi, but the relief valve showed a higher pressure (~50?).