This question has been answered several times in the past, so here it is, one more time:
The AAUS Dive Computer workshop recommends 18 hours out of the water.
At the same workshop Emmerman and Sharkey presented ways of accurately getting onto tables if you'd kept good records, that they had independently worked out (references are to the Navy tables that were in use at the time, but the same approach will work with other tables:
Record the time of day, put the computer in dive planning mode, record all the Depth/Adjusted NOD limit pairs as they scrolled by on the computer (e.g., 60 ft would permit a 22 minute NOD dive, 50 feet would permit a 31 minute
etc). Each Adjusted NOD limit would be subtracted from the NOD limit for a new diver at that depth (e.g., for 60 ft: 60 min 22 min = 38 min. For 50 ft: 100 min 31 min = 69 min
etc.) that yields a number that represents how long you would have had to dive to that depth to have the amount of nitrogen you now have in you. Then you take each Depth/Time of Theoretical Dive back to the Repetitive Group Designation Table and grab a Group Designator (e.g., for 60 ft and 38 min: Group G. For 50 feet and 69 min: Group I
etc.) Then you take the worst possible case that is to say the highest letter (e.g., I in the example we are running).
So you'd call yourself a Group I diver at the time you originally recorded, and thus be able to use the tables. The only way to get back onto a computer, however, is to follow standard computer failure protocols (which was agreed at that workshop to be no diving for 18 hours).
Not simple, but potentially useful, the full write-ups can be found in the proceedings at the RUBICON site (free, but send them money anyway, it's for the best possible cause, the advancement of human knowledge).
Other thoughts if you can't sit out for 18 hours:
Two hours of surface breathing oxygen will take you from U.S. Navy Group N to Group A.
A couple of hours with a pure oxygen rebreather (or an oxygen stage) at 15 feet would set you up even better.