You're unlikely to get more than 5-7 years from the batteries even stored as your propose.
This link describes many factors that contribute to shortened or lengthened life and notes
industry experience indicates that a 4 to 7 year VRLA battery life is more likely"
"A European study of over 1,000 installations, of various system voltages and cell
capacities, containing about 35,000 cells concluded that VRLA batteries require
replacement after 5 to 8 years of operation. The shorter lifetime (5 years) was
associated with batteries operating at 110 V or higher system voltages. The
longer lifetime (up to 8 years) was applicable to better quality batteries and those
of lower system voltages. Absorbed Glass Matt-type cells demonstrated a higher
failure rate than did gel-type cells."
These are for commercial quality batteries, it's unclear what you have, though if you know and can refer to the datasheet you'll get a more specific answer. The batteries in the study are generally used in UPSs so are kept on float charge, as you propose, except for a drain 1-2 times a year on test.
This is a link to a VRLA battery with a 20 year design life on float charge, so it is possible but again extremely unlikely this is the battery you have.