I found this Pharmacist Newsletter that provides some pretty specific info on the question...
June 2003
"Tablet Stability
Past tradition has led pharmacists to counsel patients on the “instability” of sublingual nitroglycerin (Nitrostat, NitroQuick, NitroTab). We told patients that SL NTG was easily degraded by air, moisture, and light. We told them to frequently replace opened bottles (usually every six months). Is this information correct today? No, this is NOT true today. NTG tablets have been reformulated to a stabilized formulation. These are compressed tablets; the old tablets were molded. Expiration date from the time of manufacture is 24 months.
Long-term testing (greater than 24 months) of the tablets has demonstrated excellent physical and chemical properties when exposed to continuous daily opening and closing of the container. Testing was accomplished with exposure to recommended room temperatures and humidity.1,2 It is currently recommended that patients keep the NTG in the original glass container, tightly capped. Store the tablets at room temperature, 20 ̊ to 25 ̊ C (68 ̊ to 77 ̊ F).3,4 Patients should use the tablets until the labeled expiration date is reached. The original NTG container should be used and tightly resealed after each use. Tablets are expected to retain full- labeled potency when stored under recommended conditions, even with frequent opening.1,2
Potency
Due to NTG tablet reformulation and greater stability, the newer tablets may take ten to 20 seconds longer to dissolve in the mouth. Vasodilatory effects occur approximately one to three minutes following a sublingual dose. A maximum effect is reached within five minutes after a dose, with effects lasting for at least 25 minutes.3
A burning or tingling sensation sometimes occurs following sublingual dosing. The stabilized formulation of NTG may be less likely
to produce this effect, and some patients are less sensitive to this. The burning/tingling sensation should not be used as an indicator of tablet potency.2,3,4"
ftp://host-208-105-2-96.midco.net/Seagate_Expansion_Drive/Karla%20PC%20C%20drive/NP%20School/NUR%20568/Cardio/Sublingual_NTG_and_Stability_Prescribers_Letter_20.pdf
And this...
The physical stability of one type of stabilized molded and three types of compressed nitroglycerin tablets was studied. The evaporation rate of nitroglycerin was controlled by its vapour pressure and by the matrix effect of the dosage forms. The four products showed different vapour pressures and matrix effects. In time nitroglycerin escapes from the outmost layers of the tablets; the dosage form in which the drug showed the lowest vapour pressure (the stabilized molded tablet) was found to be the most stable one. However, from the time when the drug had escaped from the outmost layers of the dosage form, the matrix effect became dominant.
When nitroglycerin tablets were stored in tightly closed containers at room temperature potency loss was minimal. Even when the bottles were opened regularly this did not result in a significant loss of the drug. Measures are suggested to minimize drug evaporation.
http://www.ncbi.nlm.nih.gov/pubmed/3145487