Heat can cause problems for containerized plants. I'd suggest that you hold off on fertilization...the combination of very high temperatures plus frequent doses of soluble fertilizer can result on the leaf burn you've observed.

hi .. welcome to GW ...

you got it last w/end.. and you have already fertilized it twice??? one might suggest you are on the verge of loving this thing to death ...

and you water it when it needs water ... insert finger.. that medium looks rather peaty to need watering every other day ... but it is really up to your finger to figure that out ....

and when i bring home a plant.. i NEVER place it out in full sun .. since i have no idea if it is acclimated for full sun .. i would give it a few days in very bright shade.. to get it hardened off... and then start moving it toward full sun ... [it looks like a greenhouse plant.. and any glass between the sun and a plant.. in theory.. is not full direct sun ..] ..

and with the plant getting rather large.. and the pot getting smaller.. i might avoid full sun altogether ... the pot just might not be holding enough water for all day in the sun ....

put it in bright shade .. water it properly.. dont fert it for at least a month.. and then very diluted.. and just enjoy it... once it goes to seed.. there wont be anymore flowers.. unless you take that one off ....

good luck.. and good day

ken

ps: dont make presumptions just on the name.. because.. in this case.. a sunflower.. probably should be in mother earth.. rather than a pot ... and that is where problems start ...

There appears to be algae or moss on the surface of the soil. You could be over-watering. And 2 doses of fertiliser in a week is OTT.

Potted sunflowers like this which are sold here are usually raised in greenhouses, so possibly yours is not used, as Ken, suggested, to full on outdoor conditions.

How long are these supposed to last?

I hope your plant survived. It is a magnificent sunflower.