Not true...there's actually two separate issues:
1) Depending on the market situation it may take multiple months to purchase the weapons/ammo. It depends simply on market shortages.
2) Issue two is military spending space. Say you buy 2,000 helicopters. It may take more than one month to actually purchase these from the market, since they are typically in shortages. However, all 2,000 are deducted from your spending space immediately. What I think you may be seeing is your spending space doesn't jump back up to the max of 750B after one month. How fast it increases depends on the economic situation of your country (and also, obviously, any new military purchases you conduct).
Kevin