It would help to supply the following:
1. Is it just a chemical reaction in a 500 mL round bottom flask or is it more elaborate?
2. Are you dealing with organolithiums or something extremely reactive or something that reacts with air, water or nitrogen at elevated temperatures?
3. How hot is the reaction - will gases be evolved - does it have to be an open system?
If you have valves on the container, apply a vacuum, seal the container, and then bleed He into the container. Repeat this three times and you are almost at 100 % He.
If you have an open system, let the He purge for 10-15 min before starting the reaction and keep a moderate flow (meaning you can displace the volume of the container in about 30 seconds - ie a 1 L container would need a flow rate of 2 L/min) of He exiting through a narrow orifice to make air diffusion back into the container minimal.
This is all relative. If the material is extremely reactive you will have to resort to "Schlenk lines" or some ridiculously elaborate setup to get decent yields and to keep things safe.