You get 1-2 frags every 60 pulls on average. Thats anywhere between 2400-4800 pulls in total. Assuming you get 1 and 2 frags at the same rate, 3200 is the best approximation. If you need 40 frags, then half of it, which is 1600. There are 5 treasures (kage, lol, edo, jin1, jin2) that offer his frags. For 60 free pulls it takes 120 days, if you divide it by the number of treasures it makes you pull itachi (1-2frag) every 24days on average. So its possible for you to get itachi entirely for free in 960 days ;)

If there's any legit advice I could give you, if you plan to keep playing this game I'd suggest going for edo hashi instead. This way you will obtain some of free Itachi frags. I assume you are about 1k seals in gnw, so you will need more or less 2k seals (which will take you lets say 1.5 year to obtain). By that time, 550days will pass which equals roughly to 22 chances of pulling ssn itachi frags. Lets asume you get 11x 2frags and 11x 1frag, which leads you to 33 out of 40 frags. Now you can save another 300 seals and fini*achi. It's up to you to decide if you want to spend 1600 now (propably 1000 or less because it will take you about a year to gather seals anyway) or add another 1000seals to it and have both instead.

Going for itachi now is a trap and worst decision you can make. trust me, I did the same mistake.

Because Itachi's spectre of use in todays meta is very limited, even more if you arent top dog of your server. ISure, if you are top power you can still make itachi buff blitz team work. However if you are in top10 with other being really close, itachi cant really shine vs those people. Also everybody nowadays either dodges or is immune, so you cant have chaos on demand. While you might block some annoying spells like madara 5k or samehada barrier, you cant interrupt move1 minato or naruto, which are the most common teams you face today. If you are slower then it doesnt matter, but if you are faster you are basically letting opponent get heavy nuke during r2 for free. If you look at ninja kit, itachi's best side is strong column attack (which hits basically 1 ninja in 1v1), 2x2round chaos and super armor with immunity. Chases are generic and dont offer much.

Hashirama on the other hand doesnt care for dodges if used from move1, has amazing 12target mystery, that can destroy barriers (although they arent as popular as they used to be). If used as a support role he offers full clear + heal to 2 most popular ninjas (naruto/minato) as well as wind/water main with senjutsu talent. Having 2 ninjas that can full clear at least 3 out of 4 ninjas in your team is really hard to deal with. On top of that he offers 2 godlike chases- immobile+full chakra drain and 2man immobile on multihit chase (which, again, our most popular ninjas achieve with ease). His only generic and not-awesome ability is his standard attack, which is single target and dmg isnt anything special.

While itachi is very flexible in 1v1 and can be used as a backline control unit, hashirama in 1v1 fits current meta better. You could potentially argue they are equally useful, but once you start using senjutsu team, hashirama is simply better in every aspect except r1 control on picked target. When it comes to 2v2 or 3v3, cross field damage and heal/clear kicks in and hashi is simply WAY better. Current meta simply favours hashi too much for itachi to even think about being competitive, and while im jonin team user myself, thats why i consider keeping going for itachi when gnw was released a big mistake. People who went gnw instead at that time are about 300 seals away from getting hashi and pretty much 5-10itachi frags away. While I have itachi, im barely 40 frags in towards hashi. And im hardly ever using Itachi anyway. Not saying hes bad, just that his time to shine is over.