The correct answer is very close to 5.5mins taking into account the population growth rate and assuming this were to happen in July of 2003.
If you go to the CIA world fact book
http://www.cia.gov/cia/publications/factbook/geos/ch.htmlyou can see that the population was 1,286,975,468. The growth rate was 0.6% annually and China shares 3645km of border with Russia.
1km = 3280.8399ft
3645km = 11958661.4355ft
11958661.4355 / 3 = 3986220.4785 people in 1 line along the border
1286975468 / 3986220.4785 = 322.8560 rows
3 feet spacing between rows as well
322.8560*3 = 968.5680 feet between the last row and the border
1mile = 5280ft, 2mph = 10560ft/hr
968.5680 / 10560 = .09172 hours = 5.5032 mins
so 5.5mins for the last row to cross into Russia.
Now to check the impact of population growth (which Ill do linearly since we are dealing with a small increment of time).
with a population of 1286975468 and a growth rate of 0.6% annually 772185280.8 ppl will be born in 1 year.
365*24*60 = 525600mins/yr
772185280.8/525600 = 1469.15ppl per minute
1469.15*5.5032 = 8080.325 ppl will be born by the time the chinks finish marching into Russia. This isnt nearly enough to complete a full row and when you take into account that half of these born wont even be chinese since they will have been born in Russia. Round my answer to 5.5mins and and impact population growth has will be too insignificant to impact the answer.
Assuming all children that are born are sent to the end of the rows to enter Russia as Chinese, China would need a growth rate of about 296%