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The HP-41 and Voyagers use masked ROMs. All plug-in ROMs that HP sold for the HP-41 used masked ROMs. Some third-party plug-ins for the HP-41 used EPROMs.
The HP 48S[X], HP 48G[X], HP 48G+, and HP 38G used masked ROMs. However, plugin cards for the SX and GX may use EPROMs.
The HP 49G and HP 49G+ use flash memory, which has roughly speaking the same data retention characteristics as EPROM.
The HP 39G, 40G, and 48GII probably use masked ROM, but it's possible that they might use OTP EPROM.
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Most EPROMS were rated for 10 years of data storage... but this was powered on at full temp storage. Storage time goes down exponetially with increased temperature. Since most EPROMS were never run under these worst case conditions, they probably have quite a bit of life in them.
That said, if you have socketed EPROMs and a device reader, make a backup copy.
I have experienced quite a bit of problems with data stored in MCM2801 EEPROMs used as configuration parameter storage in Mettler PE series scales. These were first gen EEPROMS and are now leaking their bits. Whenever I restore one of these scales, I rewrite them.
