God is faithful. If God wants you involved in giving, it's because He has good in mind for you. Many people find their eyes opened to the awesome power of God, and their faith strengthened, by trusting Him for the money to give. Work out the amount with God, and trust Him. He is faithful.
Most people give monthly, for a number of reasons.
One is that we often forget to do things if we don't make them a routine. Another is that missionaries generally receive their support checks monthly. Many others give gifts as they can, sometimes just once. Some give quarterly. Most people make a long-term commitment, but not all do.