If you want to make really thick and creamy Milk kefir then the fat content of the milk you use is important. Don't be concerned if you have just received your grains and the kefir they are producing is a bit thin. This is normal and is part of the settling in process that your grains will naturally go through. this can take a few days to a week or so but then your kefir will thicken up and will be of a pourable smoothie yogurt consistency. If once this has happened you are still looking to make even thicker kefir then there are ways to make your kefir thicker.
The fat content of the milk you use:
In general the fat content determines the thickness of your kefir so full fat (blue top) milk will give you the thickest kefir. If this kefir is still not thick enough for you then you need to add some thick heavy pouring cream to your milk. The more cream you use the thicker your kefir will be.
If you are using Raw milk then make sure you give it a good shake when you open it as this evens out the cream content, otherwise you will end up with the first batch being really thick and then you will have runny kefir. If you use pure cream then you should end up with a yogurt rather than a drink.
Ready to bottle or not that is the question
When you are looking for thick kefir the timing of bottling your kefir is REALLY important. If you bottle it too earlier it wont be as thick and if you bottle it to late ditto. The ideal time to bottle your kefir is when you see little pockets of clear whey forming in the jar. Its tricky to get that spot on but that is the ideal time to bottle if you want really thick kefir.
Another trick is to leave your kefir at room temperature in the bottle for another 12-24 hours after you have removed the grains. Keeping the drink at room temperature instead of putting it straight in the fridge means that the drink will go through a second ferment. As the grains have now been removed the second ferment will now be milder than the first and this can help to thicken kefir.
If you follow all of the above then you should be making epic creamy kefir!
just a follow on note: Kefir is generally safe to be kept in the fridge for 7-10 days but obviously use your nose. If it doesn't smell good then don't drink it!