Hi,
if no CO2 escapes to the surface and dissolves completely, then pH must go down. How fast depends on CO2 amount, water quantity and KH. Usually it takes about 24 hours to get pH stabilized.
However, if your sand includes calcite or lime, then the CO2 will keep dissolving more and more KH from the sand and that will be pushing pH up. This can take a week or so.
Interesting is, when I need to get CO2 out of water, then I have to leave a cup standing on table for 24 to 48 hours to get stable pH reading.
Edward