how soon can I put goldfish in tank?

12 hours is enough for the water to settle and degas, however you will need to upgrade the fish asap (i really mean asap), 28L is only big enough for a betta...

20 gallons for fancy goldfish (and 10 gallons foe each additional) with filtration rated for twice the tank size

50 gallons each (w/ 75 gallons being really recommended) for common/comet/shubunkin goldfish with filtration rated for twice the tank size.

small goldfish are young goldfish which will grow

Take a bag and fill it with the water your fish are in now and put the fish in it. Seal the bag. Put the whole bag (old water and fish and all) into the new water. This is supposed to let the old water get to the temperature of the new water and slowly let the fish get used to it. I have always done this and I never had a problem until the last time when I forgot and just dumped the fish in. I ended up killing four 6-inch really pretty goldfish.

The tank is way too small. Common goldfish need at least 55 gallons (about 208 liters) each when full grown and fancy goldfish need at least 20 gallons (about 78 liters) each when full grown. You need to let the water cycle in for tank for a couple of weeks before you can add fish. The only fish could put in there would be a betta and you would need a heater for your tank.

You need to cycle the tank first and that takes about 6-8 weeks. You can find the directions for this by googling it. Your goldfish is going to be too big for that tank within a month or two so I wouldn't add another one. If you have a fancy goldfish it will need a tank 4-5 times the tank that you have now. If you have a comet it will need a tank about 10 times the size you currently have. I would just get a bigger tank now so you don't have to start cycling another one in a few weeks.

I usually let the water sit for some hours if not over night then dump them in as soon as I get the fish. Never had a problem.

i have 3 gold fish and i always waited over night before putting theme in the tank

Soon the tank will be too small for whatever you put in it. Unfortunately that's a really really small aquarium.