With Forex trading you can expect to have almost immediate demand for the asset you have acquired in the past at almost any possible volume. This of course applies only to regular Forex trading conditions – sometimes Black Swan events happen on the Forex market and due to the enormous price changes it becomes quite hard to trade. The good news is that such events are quite rare.
During normal market conditions you can stay safe in the knowledge that your trades could be executed within seconds, as there is a constant demand and supply for the currency units. In Forex terms, this is known as liquidity and Forex is considered to be the most liquid market.