tl;dr If they haven’t added any spells beyond what they get from leveling up, I’d say anywhere up to 4-7 is a fair baseline, but generally I would limit it to 1st and 2nd level spells.

Given that it’s a time jump, the way I would handle it is that whatever they’re gaining is basically through downtime, not adventuring (because if there was adventuring we wouldn’t be time skipping). So then you have to decide how comfortable you are with how many, if any, magic items you want the party to be able to gain through downtime (I’m going to treat them as obtaining the spells through scrolls even though they could also get them from other spellbooks, because there’s just not much rules guidance for obtaining spellbooks, and this is a pretty big ask). Given that most downtime activities are broken down into weeks and you’re dealing with a couple years, I would probably instead take their actions by months or even seasons and multiply the results accordingly, so as to avoid spending literal hours just to adjudicate one player’s downtime.

Another thing to consider is if they found any spells they could add to their spellbook during the initial adventure. My quick and dirty guideline is +1 spell found in the world per level up, so if they haven’t added an extra 4 spells to their book so far, I’d approve 4 of their choice at a minimum. Lastly I’d consider how many magic items the party has encountered so far. If they haven’t found any scrolls or spellbooks, I’d say two 1st-level spells and maybe one 2nd-level spell would be fair compensation.

For a simpler version, you could just use the scroll prices from the back of Xanathar’s guide, and leave it up to them to come up with the money, either with what they have now or through downtime in the time skip (don’t forget that it also costs money to copy scrolls into a spellbook and there is a chance of failure).

For reference, the Xanathar’s scroll prices are:
1st-level: 75 gp
2nd-level: 150 gp
3rd-level: 300 gp