According toi this paper in Parameters it was about 14% of GDP in the '50s and Global Secutiry says it was 15-17% of GDP in the mid-80s.  Can I ask why you are curious?

Spending as a percentage of GDP does not really tell us anything.  It is basically a number politicians throw around that sounds impressive.  Spending as a percentage of government tax revenues is more meaningful for the US.  For the Soviets we would probably need to look at what percentage of their hard currency went into defense spending because for them domestic expenditure was largely a shell game and it was only their hard currency revenues that affected their overall budget.  Of course, the Soviets generated much of their Hard Currency through weapons sales and so it would have to be further broken down into hard currency revenue and then what percent of hard currency actually spent on weapons went to spending for weapons used by the Soviet military itself.  It would probably also be useful to look at how much hard currency went into spending for their research base as most of their computers and precision machinery was also purchased in the west and even though it was a capital and not a recurring expenditure every dollar of hard currency spent reduced their resources for weapons spending even more.Analyzing the Soviet economy is even more difficult than analyzing the US economy because there were many costs that were never enumerated into dollar or ruble terms.  There are many factors that brought the USSR down, and spending was surely one of them.