I just added a whole quart of motor oil to my car . How long do I have to wait to check the level of the motor oil ?

2 minutes. 

Make sure the car is level. 

Stop at ANY oil change place to see if they can find a LEAK under the engine! If you EVER used VALVOLINE, STOP NOW or even SOONER (lol)! It is an OIL that no factory on earth will USE as it DISSOLVES gaskets RUBBER intake hoses, and SEALS over time! The use of PENNZOIL ULTRA OILS like ULTRA EURO and ULTRA PLATINUM will REVERSe all the damage done by BAD oil changes for YEARS! It wil REMOVE bad tarry COATINGS on the INSIDE of the BLOCk as well!  >>>PENNZOIL will clean the OIL CONTROL RINGS that by NOW are FULL of BAKED IN TARRY SLUDGY junk and CAUSING the oil use you are trying to elimiate! BAD OILS are EVERYWHERE.  >>>get a can of AMSOIL POWER FOAM from AMSOIL PRODUCTS.com! AMSOIL is VERY POWERFUL and even BANNED frmo the stagte of CALIFORNIA< but there is NOTHING like it< EXCEPT maybe MERCURY POWER TUNE. IT DISSOVLE sludge on CONTACT. Use it to run THROUGH your intake system and PCV valve! It will ELMINATE the BAKE IN TAR within your OIL CONTROL RINGS and CLEAN the VALVE HEADS as well!  IT wil clean the THROTTLE BODY and the IDLE AIR CONTROL VALVE and ALSO the O2 sensors and clean the GUTS of your CONVERTER that is VERY VERY PRICEY! >>>>NOT ALL OILS ARE THE SAME and EACH ca maker APPROVES or DISAPPROVES of each oil they>ST< so look for the oils with matching SPEC NUMBERS< not VISCOSITY RATINGS ont he LABEL! Audi need 502/505 oil FULL EURO SYNTHETIC ONLY<> for example! BMW needs LONG LIFE< LL-02 oil ONLY< etc and GM needs DEXOS or BETTER OIL ONLY< etc! CARS today are more like NASA technolgoy as they get MORE and MORE computers to run things on them! The BEST cars have the BEST conputers! TODAYS cars make VERY LITTLE emissions compared to a few decades ago! EXCEPT is t seems FOR GM cars and trucks! They JUST can get it RIGHT< ever! They are STILL involved in BILLION DOLLAR LAWSUITS for bad AIR BAGS and ignition switches that CATCH ON FIRE and KILL animals and harm children! ENOUGH of this lecture! YOUR ENGINE HAS CLOGGED OIL CONTROL RINGS most lilkely and YOUR OIL CHOICES are the REASON for your TROUBLES here! I use a FIFTY  FIFTY mixture of GUNK ORIGINAL ORANGE engine cleaner and AMSOI PWOER FOAM in the CRANKCASE to get nearly ALL the BAKED IN GARBAGE out of the ENGINE before a properr FULLY SYNTHETIC OIL CHANGE and the use of PENNZOIL ULTRA continues the CLEAN as you DRIVE! PISTON RINGS and VALVE HEADS! OUR O MODERN FUE<L are EUROPEAN based LOW SULFUR fuels to keep the AIR CLEAN and our ENGIENS clean inside! It is ALWAYS THE AIR intake side of engiens that CAUSE TROUBLES and the use of INFERIOR OILS with IMPOROPER SPEC NUMBERS! PENNZOIL RULES 

It shouldn't take but a minute or so to settle in the crankcase. Oil circulates being pumped to the top and it runs back down to the sump by gravity. That doesn't take long.

I wouldn't wait. Pull the dipstick out, wipe it down, put it back in, pull out and check. 

Wait at least 15 minutes. Make sure the vehicle is parked on a level surface. 

The distance from the bottom dipstick mark to the upper dipstick mark is one quart of oil. If you didn't have oil on the dipstick you're going too long between oil changes and going to ruin your motor. After adding oil just let things sit for a minute to let the oil drain into the oil pan and then pull the dipstick...wipe it off and put it all the way back in the dipstick tube to take another reading. Check your oil level and top it up every 1,000 miles between oil changes.

Drink a soda and measure the level again when you finish.

Depends on the viscosity, but it should settle fairly quickly

At room temperature oil flows almost as fast as water.

If you want to be exact, I'd wait 10-30 seconds to make sure most of the oil drops into the oil pan before measuring.