In my experience, half an hour at room temperature will make absolutely no difference. Boehringer Mannheim (now part of Roche), who at one time supplied the best NADPH, used to recommend storage at 4o C.
By A340 callibration, NAD(P)H is typically about 85% 'pure' based on dry weight measurements, and Sigma will try to tell you that the remainder is mainly water. That is, if you calculate a concentration based on the dry weight, the A340 will be about 85% what you expect.
Even if some water is absorbed from the atmosphere, this will make little difference as you are probably going to calculate the concentration based on
As User 243 has pointed out, if you are concerned that the NAD(P)H is degraded or oxidized, then an A340
measurement will probably set things right.
If you are concerned that all the A340 absorbing material is not NAD(P)H, or that a significant proportion is in the α-form (Oppenheimer & Kaplan, 1975) or is otherwise enzymically inactive, the best thing is an enzymic calibration.
This is done by adding limiting amounts of NAD(P)H to an assay system containing excess substrate and calibrating enzyme, and measuring the total decrease in absorbance (Racker, 1957).
A popular calibrating enzyme for NADH is aldehyde dehydrogenase (EC 18.104.22.168), as the reaction is practically irreversible, and it makes it easy to 'drive' the calibrating reaction to completing (and you don't have to worry about equilibria).
As I said in a previous post, NAD(P)H is UNSTABLE IN ACID (Oppenheimer and Kaplan, 1974). To put it crudely, 'the head (nicotinamide moiety) falls off', causing a bleaching of the absorbance at 340nm. Making 100mM acetate, pH 5, 100 micromolar in NAD(P)H produces a very significant decrease in A340 absorbance with time (which may be very easily monitored in a spectrophotometer).
Just because all the A340 material is due to NAD(P)H, this does not necessarily mean that the commercial preparation is pure. In fact, far from it. Dalziel (1963) showed that commercial preparations of NADH then available contained a competitive inhibitor of many dehydrogenases, most probably adenosine diphosphate ribose.
Accepting Dalziel's value of 17 600 M-1cm-1 for the extinction coefficient for NADH at 260nm, and assuming that NADPH has an identical value, then highly purified samples of NAD(P)H should have an A260/A340 ratio of about 2.83, and values higher than this may indicate the presence of impurities that absorb at 260nm, but not at 340nm. ADP-ribose is a prime example, but NAD(P)+ is another possibility.
I notice that in one of the links given by AliceD, Sigma quote a A260/A340 ratio of 2.32. So is the higher value due to the presence of residual NAD(P)+, or something else (a breakdown product?), or both?
You may need to use chromatography, typically on an anion-exchange resin, to obtain NAD(P)H free of impurities.
Dalziel, K. (1963). The purification of nicotinamide adenine dinucleotide and the kinetic effects of nucleotide impurities. J. Biol. Chem. 238, 1538 - 1543. [pdf]
Oppenheimer, N. J. & Kaplan, N. O. (1974). Structure of the primary acid rearrangement product of reduced nicotinamide adenine dinucleotide (NADH). Biochemistry 13, 4675 - 4685.[pubmed]
Oppenheimer, N. J. & Kaplan, N. O. (1975). The alpha, beta-epimerization of reduced nicotinamide adenine dinucleotide. Arch. Biochem. Biophys.166, 526 - 535.
Racker, E. (1949). Aldehyde dehydrogenase, a diphosphopyridine nucleotide-linked enzyme. J. Biol. Chem. 177, 883 - 892. [pdf]
Racker, E. (1957). Spectrophotometric enzymatic methods for the determination of aldehydes and ketoaldehydes. Methods Enzymol. III, 293 - 296.
Ciotti, M. M. & Kaplan, N. O. (1957). Procedures for determination of pyridine nucleotides. Methods Enzymol. III, 890 - 899
Horecker, B. L. & Kornberg, A. (1948). The extinction coefficients of the reduced band of pyridine nucleotides. J. Biol. Chem. 175, 385 - 390. [The definitive determination of the extinction coefficient at 340nm] [pdf]
Kornberg, A. & Pricer, W. E. (1950). On the structure of triphosphopyridine nucleotide. J. Biol. Chem. 186, 557 - 567.
[This paper shows that the 'extra' phosphate in NADP(H) is attached to the 2' position of a ribose. In Coenzyme A, of course, the phosphate attached to the ribose is on the 3' position. [pdf]
Pullman, M. E., San Pietro, A. & Colowick, S. P. (1954). On the structure of reduced diphosphopyridine nucleotide. J. Biol. Chem. 206, 129 - 141 [pdf]