Tektronix Applications Expert

It's a common practice to run Dark-level compensation immediately prior to any testing. Typically you want your equipment to be powered on for at least 20 minutes to allow any temperature induced offsets equalize, then run Dark-level compensation, then proceed with taking measurements. Some measurements are very sensitive to Dark-level compensation. I don't think that there is going to be so much a time interval related answer but rather temperature related. Here's an application note that briefly describes this: https://download.tek.com/document/85W_15763_0.pdf. And the excerpt I'm referring to:
"Dark Level Compensation must be run every time the instrument’s temperature changes. For sensitive measurements, the Dark Level Compensation
should be run whenever the settings of the instrument are significantly
changed. This is critical for higher values of Extinction Ratio (cca over 10). For
the most accurate results, the Dark Level Compensation should be run with
horizontal SCALE (time/div) and vertical SCALE (µW/div) set to the same ranges
at which the measurements will be performed. Alternately, the signal level can
be monitored when no light is connected, and the Compensation run only
when the offset level exceeds a desired limit."

Tektronix recommends once a year for Performance verification, but that is up to the customers and use case. Find more information at https://www.tek.com/en/services/repair/warranty-status-search

Here's a link to information that might help: https://www.tek.com/en/products/oscilloscopes/5-series-mso

The AFG31000 can create this signal. Link here : https://www.tek.com/en/products/signal-generators/arbitrary-function-generator/afg31000?utm_campaign=kei-afg&utm_content=now-you-can&utm_medium=display&utm_source=google&utm_term=rt

Unfortunately the P6015 used Freon as fluid which is now illegal and hasn't been available for years. The newer generation P6015A probe uses a die-electric gel but it's not possible to use this gel on the older P6015 probe.

Contact our Service Department and they can help you out with the Recall of the TDS3BATB.

To set up a repair RMA (model and serial number)
call 1-800-833-9200 opt 2.
Or send an email to rmarequest@tektronix.com
https://www.tek.com/en/services/repair/mytek_rma_request

https://www.tek.com/en/support/faqs/how-replace-cmos-battery-dpo-dsa-7000-70000-series-oscilloscopes

It's unlikely you'll be able to upgrade unless you're still in the return period. You can put 2 or 3 of these units in parallel which would solve your problem. You'd need to purchase a 2260-006 cable to put 2 in parallel or a 2260-007 cable to put 3 in parallel.

Here's a link for CMOS battery replacement: https://www.tek.com/en/support/faqs/how-replace-cmos-battery-dpo-dsa-7000-70000-series-oscilloscopes

This USB3.0 active copper solution works well: https://www.icron.com/products/icron-brand/usb-extenders/cat5/usb-3-0-spectra-3001-15/