Kerr, the all-time top-scorer in both the Australian and American leagues, joined Chelsea at the start of January and made her WSL bow six days later in the win over Reading. The forward, 26, then opened her Chelsea account in the 4-1 win over Arsenal, who the Blues meet on Saturday in the Continental Cup Final.
Speaking before the first domestic final of the season, Hayes warned it could be the start of next term before Kerr is at her best.
"I think six months, if I’m honest," Hayes said. "I think it's an adjustment – you’ve got to realise she had no pre-season. Coming in and, boom, playing in a game six days later with not much behind her belt. Got the flu, had a little bit of a quad injury, then goes away, comes back.
"This is the first week we’ve had her train with the team, so I think it’s going to take times. She goes away again on international duty, so I think it’ll take the remainder of the season, if I’m honest.
"She got the flu as soon as she got here and put it this way: I don’t know how she got out of bed to play Arsenal. But she did, so that’s a credit to her, but I think Sam Kerr is at about 60 per cent."
Kerr, who split her playing time between Perth Glory and Chicago Red Stars before joining Chelsea, is one of the biggest names in the women's game but Hayes added: "There's no superstars here. Just so I’m clear: none, including Sam Kerr.
"A wonderful individual who will fit into the team, and I’ll say that because it’s the one thing she dislikes the most. She wants to be seen as a team player, and that fits perfectly with us, too."
Kerr is expected to be partnered by Lionesses forward Bethany England against the Conti Cup holders and Hayes has hailed the 25-year-old for her perseverance in breaking into the first team.
"She’s had to earn it," Hayes said. "Has it been easy? We had to have difficult conversations, yes. Has she had to earn my trust a million percent, and that’s a real credit to her because rarely do you see that. I don’t see it enough in players.
"The next challenge for Beth is to put herself in the next echelons at international level, Champions League level, and I know she’s firmly focused on trying to do that."
Nottingham Forest's City Ground is the venue for Saturday's final and Hayes has revealed her admiration for Brian Clough, the legendary former Forest boss who guided the club to back-to-back European Cup wins in the 1979 and 1980.
"I just knew the minute we got [through], I thought, 'Oh god I’m going to Cloughey’s home'," Hayes said.
"My father loves him, and watched his teams a lot, and live. I went to a lot of Nottingham Forest games live when they were in London, so I’ve grown up always admiring his football, the type of football and he’s straight-talking, I guess. I was only a child, but was very influenced by my father.
"Him and his assistant [Peter Taylor], we’d talk a lot about in my household. Yes, I was a massive fan of Brian Clough and his team. I’ve been there as an assistant coach, but to lead my team out there is a really, really, really nice moment.
"I always think 'outspoken' gets misused. I find it refreshing to hear some honesty, his honesty in a time where there probably was more honesty than there is now with managers. Let’s not forget he produced some brilliant football teams and is most remembered for that, I hope."