There's no requirement for the time points to be equally spaced, any more than there is in an ordinary linear regression: the only drawback will be that the regression is likely to be more heavily influenced by any time points that are positioned well away from the majority, i.e. in your case the ones to the right of the graph. You might be able to mitigate the effects of that by logging your time variable before performing the analysis so that the points are more evenly spaced along your time axis; that is, you'd end up with a nonlinear equation relating voltage to log(t) as opposed to just t. It needn't necessarily be a log: any transformation that linearized the scale would do, so for example if you were recording data at 1, 4, 9, 16, 25, 36 etc seconds, a square root would be the obvious one. You might also want to consider transforming your dependent variable (i.e. the voltage) if its variability seems to be dependent upon time in some way, however you've already noted that the error bars don't seem to be changing much, so that probably isn't necessary.