I’m not sure how long it took to hold my tongue in place, but it took me 3-6 months to see initial external results, given regular mewing + chewing hard gum (falim/mastic). The bone movement occurs in tiny increments of course, but over months the changes are clear. Keeping the mouth closed while breathing is also very important, as it’s the balance of forces between the tongue and cheeks/lips that shapes the arch. For me this wasn’t easy/comfortable prior to expanding my airway via mewing. As for discomfort, I would just keep at it for a few months and see whether your palate is opening up.