The DAG file changes size at each new level. A new level is created after a total of 30,000 blocks have been mined. The increasing speed of the size evolution of the DAG file depends directly on the time it takes to find a block. However the extraction time of a block is not stable, it can vary between 10 and 60 seconds for a digital currency such as Ethereum. Thus, the time taken by each level is not defined in terms of duration, but in terms of blocks. On average, the DAG file increases by a factor of 0.72 per year for a currency like Ethereum. So, remember that it depends on the search time of a block. For other virtual coins with block that takes longer to be generated, this search time of the DAG files increases more slowly and the graphics card can work longer.
At a certain point, the currency changes from PoW (Proof of Work) to PoS (Proof of Stake). Graphics cards, then, become obsolete and we should continue to mine with the help of ASICs.