Exposure limits aren't really as simple as what volume of a chemical you can use in a day. The simplest answer is that you should reduce your exposure to ANY chemical to the lowest amount possible, with proper ventilation and personal protective equipment (goggles, gloves, respirator).
Every chemical you use should have a Safety Data Sheet (SDS) available from the manufacturer. The exposure limit for inhalation will be listed on there as a concentration in air in parts per million (ppm) as either a TLV (Threshold Limit Value) or PEL (Permissible Exposure Limit). These values are the concentration that you could be exposed to --if you worked 8-hours a day, day after day, over a work lifetime--without seeing health effects. Not every chemical has a TLV or PEL. Also, there's no real way to know what concentration is in your work space without instrumentation as it depends on the ventilation, your work practices, evaporation rate of the chemical, etc.
The lower the TLV, the more toxic the chemical. So for example both Odorless Mineral Spirits and Turpentine have TLVs=100ppm, whereas ethanol is 1000ppm.
The SDS will also include any information on health risks associated with the chemical (neurological, reproductive, carninogenicity). However, it will be based on available data, and for a lot of chemicals there isn't enough research. So, just because a health effect isn't listed, doesn't mean there isn't one---could be there just isn't any data available.
As for oil of spike lavendar, there isn't a TLV value and it looks like different manufacturers have different formulations. The one from DickBlick states: "High density vapor irritates respiratory organ, and induces dizziness,
headache, anesthesia, slumber or the effect to central nervous system.
Spirit of petroleum: Chronical excess inhalation in long time of mist induces
inflammation of lung, and may cause fibrosis on pulmonary artery." Which would pretty much keep me from using it in any situation were there wasn't proper ventilation. Another lists limonene as a component, which is a controversial chemical because it's marketed as a "safe and natural" alternative. There is no US exposure value, but European values are 5ppm (remember turpentine is 100ppm and lower is more toxic.) There are lots of "natural" things that are poisonous.
Inhalation is just one route of exposure (probably the most relavant for artists if you're just using artist's materials in a normal way), but ingestion and absorption are other routes, which is why personal protection and proper work practices are so important. There's no way to know if one particular person will develop a health problem or solvent sensitivity when another will not.