It depends on what coin you are going to mine. If it is ETH then you should know that there are a lot of predictions about dag file increasing. Someone says that dag file size will increase to 4 GB at September of 2018, someone that file will increase at March of 2019. The middle is the end of 2018. About other coins I don't know. You can find information in Google.
https://www.reddit.com/r/EtherMining/comments/7dzqfa/eth_mining_future_dag_size/