Time series analysis on small dataset
There is no valid answer. If you have a stationary process with 0 variance, then the forecast horizon has no limit.

More realistically, you have the follow rule of thumb (which is totally from experience with absolutely no theoretical base): the horizon forecast may be half the historical base. You have 8 data points, so you can forecast 4 points. You have 2 years, so you can forecast 1 year.

The key point is : check this after your forecast is done.