How to extract thymol from oregano leaves?
This sounds like a nice project and I'm tempted to answer in too much detail, but since this is homework [&hellip;]. Here are some hints:


Find information about thymol. This is crucial! 


What is the structure?
In which solvents is it soluble? 
Is thymol soluble in water? 


Is it soluble in acidic or alkaline solution?

What about the boiling point and the melting point?

Plant leaves have a cell structure in which your thymol is somehow bound and hidden. In order to break of the structure, you may want to 


dry and powderize the leaves or
cut the fresh leaves into pieces and grind them in a mortar with sea sand

Oregano leaves are green. You know that they contain chlorophyll. There's probably a lot of other compounds too, not just thymol.
Supposed that you treat the leaves as described in 2. and start to extract using ethanol in a Soxhlet extractor, you'll end up with a nice cocktail. Do you need to separate thymol from the other extracted compounds? If so, how will you proceed? Knowing the properties of thymol will help you here!


Remember that the easiest work is the one that you can avoid ;-)

Think in analogies. Thymol is a monoterpene. A lot of other members from this family have a typical smell and are used as fragrances. There must be an established, mild procedure to obtain these compounds from plant material.


   Yes there is! Search for steam distillation :)


If there is an established prodecure that avoids column chromatography or other purification techniques, maybe somebody has already used it for thymol and wrote about it.

Back in the days, our professors used to say with a smile:


  A week in the lab saves you an hour in the library 


Replace library with internet and have another search for a suitable and easy procedure already published.



Update

In a follow-up comment, you asked:


  Then, how much ethanol should I use?


My suggestion is: Avoid subsequent laborious workup and do not use any ethanol at all! 

Read Introducing Organic Chemistry Students to Natural Product Isolation Using Steam Distillation and Liquid Phase Extraction of Thymol, Camphor, and Citral, Monoterpenes Sharing a Unified Biosynthetic Precursor by Katherine A. McLain, Kenneth A. Miller, and William R. Collins, J. Chem. Educ., 2015, 92, 1226-1228. Follow the given procedure, which was deliberately designed to be performed in a students' lab:


  One, 3 h laboratory period is required to complete the experiment. Dried plant material (10 g) or fresh plant material (20 g) is placed in a high-speed blender with distilled water (200 mL) and blended to a fine
  slurry. The slurry is transferred to a round-bottom flask (500 mL) with a distillation apparatus attached and a stir bar, and it
  is placed directly on a hot plate turned to high heat. A beaker
  (200 mL) is used as the receiving vessel to collect
  approximately 100 mL of distillate. The bioactive molecule is
  extracted from the distillate with an appropriate organic solvent
  (30 mL). The organic phase is dried with magnesium sulfate,
  and the organic solvent is removed on a hot plate at medium
  heat. 


For further details, have a look at the Supporting Info.