What is the test-retest improvement in Block Design test?
Ronnlund and Nilsson (2006) studied performance on the WAIS block design test in a large sample of adults both longitudinally and cross-sectionally. The time difference was about 5 years. They wanted to tease out age related changes from practice effects. I believe they estimated the 5-year practice related improvement to be about 0.6 of a T score (i.e., about .06 of a standard deviation).

Benefits of practice presumably decay as a monotonically decreasing function of time between practice and retest. Thus, presumably, any practice effects would be a fair bit greater at two months than they would be at five years.

Dikmen et al (1999) performed a study involving 384 adult participants who completed several test including the WAIS Block Design on two occasions separated by between two and twelve months.
Specifically see Table 2 that reports time 1 and time 2 results:

Time 1 M (SD): 11.3 (3.0)
Time 2 M (SD): 12.0 (3.2)


Thus, the standardised increase was $d = 0.7 / 3.0 = 0.23$.

The study also reports in Table 5 practice effects for various test-retest intervals (less than 3 months, 4 to 9 months, and > 10 months). See Table 5 for more information, but in general greater practice effects were observed for WAIS performance IQ for shorter time periods. Thus, you might expect, given your two month period a slightly larger effect than the overall effect (perhaps $d=.30$ or some such).

References


Dikmen, S.S., Heaton, R.K., Grant, I., Temkin, N.R. &amp; others (1999). Test--retest reliability and practice effects of expanded Halstead--Reitan Neuropsychological Test Battery. Journal of the International Neuropsychological Society, 5, 346-356.
Ronnlund, M. &amp; Nilsson, L.G. (2006). Adult life-span patterns in WAIS-R Block Design performance: Cross-sectional versus longitudinal age gradients and relations to demographic factors. Intelligence, 34, 63-78.